L 


EARTH   SCULPTURE 


OR 


THE  ORIGIN    OF   LAND-FORMS 


BY 


JAMES  GEIKIE,  LL.D.,  D.C.L.,  F.R.S.,  ETC. 

U* 

MURCHISON    PROFESSOR   OF   GEOLOGY  AND    MINERALOGY 
IN   THE   UNIVERSITY   OF   EDINBURGH  ;   FORMERLY   OF   H.M.    GEOLOGICAL  SURVEY 

AUTHOR  OF  "THE  GREAT  ICE  AGE,"  "PREHISTORIC  EUROPE,"  ETC. 


ILLUSTRATED 


wr  THE 

UNIVERSITY, 

OF 
122 

NEW   YORK 

G.  P.  PUTNAM'S   SON.S 

LONDON 

JOHN   MURRAY 

IQ08 


COPYRIGHT    1898 

BV 
G.  P.  PUTNAM'S  SONS 


Ube  Itntcherbocher  fnccee,  «ew 


PREFACE 

A  LTHOUGH  much  has  been  written,  especially 
**  of  late  years,  on  the  origin  of  surface-features, 
yet  there  is  no  English  work  to  which  readers  not 
skilled  in  geology  can  turn  for  some  general  account 
of  the  whole  subject.  It  is  true  that  all  geological 
text-books,  and  many  manuals  of  geography,  devote 
some  space  to  its  discussion,  while  not  a  few  excellent 
treatises  deal  at  large  with  one  or  more  of  its  sub- 
divisions. Geological  literature  is  also  by  no  means 
poor  in  admirable  popular  monographs  descriptive  of 
the  geology  and  geography  of  particular  regions,  in 
which  the  origin  of  their  surface-features  is  more  or 
less  fully  explained.  But  for  those  who  may  be  de- 
sirous of  acquiring  some  broad  knowledge  of  the 
results  arrived  at  by  geologists  as  to  the  development 
of  land-forms  generally,  no  introductory  treatise  is 
available.  Possibly,  therefore,  the  present  attempt 
to  supply  a  deficiency  may  not  be  wholly  unaccept- 
able. 

In  a  work  addressed  more  particularly  to  non-spec- 
ialists, technical  terminology  should  be  employed  as 
sparingly  as  possible,  and  I  have  consequently  made 
scant  use  of  those  neologisms  in  which,  unfortunately, 


18975 


iv  PREFACE 

the  recent  literature  of  the  subject  too  much  abounds. 
Technical  words  and  expressions  cannot,  however,  be 
entirely  dispensed  with,  but  those  which  my  readers 
will  encounter  have,  as  a  rule,  been  long  current,  and 
few  are  likely  to  be  unfamiliar. 

The  materials  used  in  the  preparation  of  this  book 
are  for  the  most  part  from  the  common  stock  of  geo- 
logical knowledge,  and  it  has  not  been  thought  neces- 
sary, therefore,  to  burden  the  pages  with  references. 
Those  who  would  pursue  the  subject  further  must 
consult  the  larger  text-books  of  geology  in  English, 
French,  and  German,  which  usually  indicate  the  more 
notable  sources  of  information.  The  following  works 
will  also  be  found  very  helpful  as  guides  and  instruct- 
ors : — 

Sir  A.  C.  Ramsay's  Physical  Geology  and  Geography 
of  Great  Britain. 

Prof.  A.  H.  Green's  Physical  Geology  (chap.  xiii.). 

Sir  A.  Geikie's  Scenery  and  Geology  of  Scotland. 

Prof.  E.  Hull's  Physical  Geology  and  Geography  of 
Ireland. 

Sir  J.  Lubbock's  Scenery  of  Switzerland  and  the 
Causes  to  which  it  is  Due. 

Dr.  E.  Fraas's  Scenerie  der  Alpen. 

Major  J.  W.  Powell's  Canyons  of  the  Colorado. 

MM.  De  la  Noe  and  Emm.  de  Margerie,  Les 
Formes  du  Terrain — an  admirable  and  well  illustrated 
work,  descriptive  of  the  geological  origin  of  land- 
forms. 

Prof.  A.  Penck's  Morphologic  der  Erdoberflache — a 


PREFACE  v 

masterly  review  and  classification  of  the  surface-feat- 
ures of  the  earth,  with  a  full  discussion  of  their  origin. 
This  treatise  is  particularly  rich  in  references  to  the 
literature ;  the  whole  history  of  geological  opinion  on 
the  subject  of  which  it  treats  may  therefore  be  gath- 
ered from  its  pages. 

Prof.  A.  de  Lapparent's  Lefons  de  Geographic  Phys- 
ique— a  most  instructive  and  comprehensive  outline 
of  geo-morphology.  The  second  half  of  the  work 
deals  more  particularly  with  geographical  evolution, 
the  special  treatment  of  which  does  not  come  within 
the  limits  of  my  essay.  This  interesting  subject  has  of 
late  years  been  studied  with  great  assiduity,  especially 
by  Prof.  W.  M.  Davis  and  others  in  North  America. 

The  maps  and  sections,  and  the  monographs,  me- 
moirs, and  reports  of  our  own  and  other  national 
geological  surveys  are  storehouses  of  information 
and  instruction  in  physiographical  geology.  Some 
of  these  works  that  deal  more  especially  with  denuda- 
tion and  the  relation  of  surface-features  to  geological 
structure  have  indeed  become  classical.  Amongst 
these  are  Ramsay's  notable  paper,  "  On  the  Denuda- 
tion of  South  Wales  and  the  Adjacent  Counties  of 
England "  (Memoirs  Geological  Stirvey  of  England, 
vol.  i.,  1846)  ;  Heim's  Mechanismus  der  Gebirgsbil- 
dung,  etc.  (which,  although  an  independent  work,  was 
yet  commenced  under  the  auspices  of  the  Swiss  Geo- 
logical Commission)  ;  Dutton's  "Tertiary  History  of 
the  Grand  Canon  District "  (Monograph  II.  of  U.  S. 
Geological  Survey). 


vi  PREFACE 

For  the  use  of  several  illustrations  (Figs.  8,  25,  26, 
75,  78)  from  Major  Powell's  Canyons  of  the  Colorado, 
I  am  indebted  to  his  publishers,  Messrs.  Flood  & 
Vincent.  I  am  under  similar  obligations  to  the  Coun- 
cil of  the  Geological  Society  for  a  section  (Fig.  34) 
borrowed  from  my  brother's  paper  on  the  North-west 
Highlands  ;  to  Mr.  Stanford  for  reproductions  of  il- 
lustrations (Figs.  77,  83,  87,  88)  from  my  Outlines 
of  Geology  ;  to  Herr  Tempsky,  Vienna,  for  Figs.  41, 
45,  56,  from  KirchhofFs  Lander kunde  des  Erdteils 
Europa ;  and  to  my  friend,  Mr.  W.  E.  Carnegie 
Dickson,  for  the  photographs  reproduced  on  Plates 
I.  and  II. 

EDINBURGH,  July  i,  1898. 


CONTENTS 

CHAPTER    I 

PAC« 

INTRODUCTORY     i 

Early  views  as  to  origin  of  Surf  ace -features — Rocks  and  Rock- 
structures — Architecture  of  the  Earth's  Crust — General  evidence  of 
Rock-removal. 

CHAPTER  II 
AGENTS  OF  DENUDATION 18 

Chemical  composition  of  Rocks — Epigene  Agents — Insolation  and 
Deflation — Chemical  and  mechanical  action  of  Rain — Action  of 
Frost ;  of  Plants  and  Animals ;  of  underground  Water ;  of  Brooks 
and  Rivers — Rate  of  Denudation — Denudation  and  Sedimentation 
go  hand  in  hand. 

CHAPTER  III 

LAND-FORMS  IN  REGIONS  OF   HORIZONTAL  STRATA        »      44 

Various  factors  determining  Earth  Sculpture — Influence  of  Geo- 
logical Structure  and  the  Character  of  Rocks  in  determining  the  Con- 
figuration assumed  by  Horizontal  Strata — Plains  and  Plateaux  of 
Accumulation. 

CHAPTER  IV 

LAND-FORMS  IN    REGIONS  OF  GENTLY  INCLINED  STRATA       73 

Escarpments  and  Dip-slopes — Dip-valleys  and  Strike-valleys — 
Forms  assumed  by  a  Plateau  of  Erosion — Various  directions  of  Es- 
carpments— Synclinal  Hills  and  Anticlinal  Hollows — Anticlinal 
Hills. 

vii 


viii  CONTENTS 

CHAPTER  V 

PACK 

LAND-FORMS  IN  REGIONS  OF    HIGHLY  FOLDED  AND  DIS- 
TURBED STRATA     . 92 

Typical  Rock-structures  in  Regions  of  Mountain-uplift — General 
Structure  of  Mountains  of  Upheaval — Primeval  Coincidence  of  Un- 
derground Structure  and  External  Configuration — Relatively  weak 
and  strong  Structures — Stages  in  the  Erosion  of  a  Mountain-chain — 
Forms  assumed  under  Denudation — Ultimate  face  of  Mountain-chains. 

CHAPTER  VI 

LAND-FORMS  IN  REGIONS  OF  HIGHLY  FOLDED  AND  DIS- 
TURBED STRATA  (continued")  .         .         .         .         .         .128 

Structure  and  Configuration  of  Plateaux  of  Erosion — Forms  as- 
sumed under  Denudation — Mountains  of  Circumdenudation — His- 
tory of  certain  Plateaux  of  Erosion — Southern  Uplands  and  Northern 
Highlands  of  Scotland — Stages  in  Erosion  of  Table-lands. 

CHAPTER  VII 
LAND-FORMS  IN  REGIONS  AFFECTED  BY  NORMAL  FAULTS 

OR  VERTICAL  DISPLACEMENTS 150 

Normal  Faults,  general  features  of — Their  connection  with  Folds 
— Their  origin — How  they  affect  the  Surface — Faults  of  the  Colorado 
region,  and  of  the  Great  Basin — Depression  of  the  Dead  Sea  and  the 
Jordan — Lake  Depressions  of  East  Africa — Faults  of  British  Coal- 
fields— Bounding  faults  of  Scottish  Highlands  and  Lowlands — Fault- 
bounded  Mountains — General  conclusions. 

CHAPTER  VIII 
LAND-FORMS  DUE  DIRECTLY  OR  INDIRECTLY  TO  IGNEOUS 

ACTION   ..........     173 

Plutonic  and  Volcanic  Rocks — Deformation  of  Surface  caused  by 
Intrusions — Laccoliths  of  Henry  Mountains — Volcanoes,  Structure 
and  Form  of — Mud-cones  —  Geysers — Fissure-eruptions  —  Volcanic 
Plateaux — Denudation  of  Volcanoes,  etc.,  and  resulting  features. 

CHAPTER  IX 

INFLUENCE   OF   ROCK   CHARACTER   IN    THE   DETERMINA- 
TION OF  LAND-FORMS     .......     195 

Joints  in  Rocks  and  the  part  they  play  in  determining  Surface- 
features — Texture  and  Mineralogical  composition  of  Rocks  in  rela- 
tion to  Weathering — Forms  assumed  by  various  Rocks. 


CONTENTS  ix 

CHAPTER  X 

PAGE 

LAND-FORMS  MODIFIED  BY  GLACIAL  ACTION     .        .        .     212 

Geological  action  of  existing  Glaciers — Evidence  of  Erosion — 
Origin  of  the  Ground-moraine  :  its  independence  of  Surface-moraines 
— Infraglacial  smoothing  and  polishing,  crushing,  shattering  and 
plucking — Geological  action  of  Prehistoric  Glaciers — General  evi- 
dence supplied  by  Ancient  Glaciers  of  the  Alps. 

CHAPTER  XI 
LAND-FORMS  MODIFIED  BY  GLACIAL  ACTION  (continued)  .     232 

Former  Glacial  conditions  of  Northern  Europe — Extent  of  the  old 
Inland  Ice — Glacial  character  of  Boulder-clay — Central  Region  of 
Glacial  Erosion  and  Peripheral  Areapf  Glacial  Accumulation — Fluvio- 
glacial  deposits — Loess,  origin  of  its  materials — Glaciation  of  North 
America — Modifications  of  Surface  produced  by  Glacial  Action. 

CHAPTER  XII 
LAND-FORMS  MODIFIED  BY  ^OLIAN  ACTION      .         .         .     250 

Insolation  and  Deflation  in  the  Sahara — Forms  assumed  by  Gran- 
itoid Rocks  and  Horizontal  and  Inclined  Strata — Reduction  of  Land- 
surface  to  a  Plain — Formation  of  Basins — Dunes  of  the  Desert — 
Sand-hills  of  other  regions — Transport  and  Accumulation  of  Dust — 
Loess,  a  dust  deposit — Lakes  and  Marshes  of  the  Steppes. 

CHAPTER  XIII 

LAND-FORMS     MODIFIED    BY    THE    ACTION    OF     UNDER- 
GROUND WATER .     266 

Dissolution  of  Rocks — Underground  Water-action  in  Calcareous 
lands — Karst-regions  of  Carinthia  and  Illyria — Effects  of  Superficial 
and  Subterranean  Erosion — Temporary  Lakes — Caves  in  Limestone 
— Caves  in  and  underneath  Lava — "  Crystal  Cellars  " — Rock-shelters 
— Sea-caves. 

CHAPTER  XIV 

BASINS 278 

Basins  due  to  Crustal  Deformation — Crater-lakes — Dissolution 
Basins — Lakes  formed  by  Rivers — ^olian  Basins — Drainage  dis- 
turbed by  Landslips — Glacial  Basins  of  various  kinds  ;  as  in  Corries, 
Mountain-valleys,  Lowlands,  and  Plateaux — Ice-barrier  Basins- 
Submarine  Basins  of  Glacial  Origin. 


CONTENTS 
CHAPTER  XV 


PAGE 


COAST-LINES .315 

Form  and  general  trend  of  Coast-lines — Smooth  or  Regular  Coasts 
— Influence  of  Geological  Structure  on  various  forms  assumed  by 
Cliffs — Cliffs,  cut  in  Bedded  and  in  Amorphous  Rocks — Sea-caves — 
Flat  Coast-lines  and  Coastal  Plains — Indented  or  Irregular  Coasts — 
General  trends  of  Coast-lines  determined  by  form  of  Land-surface — 
Subordinate  Influence  of  Marine  Erosion. 

CHAPTER  XVI 
CLASSIFICATION  OF  LAND-FORMS 335 

Plains  of  Accumulation  and  of  Erosion — Plateaux  of  Accumulation 
and  Erosion — Hills  and  Mountains  :  Original  or  Tectonic,  and  Sub- 
sequent or  Relict  Mountains — Valleys  :  Original  or  Tectonic,  and 
Subsequent  or  Erosion  Valleys — Basins — Coast-lines. 

CHAPTER  XVII 
CONCLUSION         •  '„.-'•        * 364 

The  study  of  the  Structure  and  Formation  of  Surface-features  prac- 
tically involves  that  of  the  Evolution  of  the  Land. 

APPENDIX     .        .        .        .        .        .        .        .  .    373 

GLOSSARY "* .         .        .    375 

INDEX •        .     387 


LIST  OF  ILLUSTRATIONS 


FIGURE  PAGE 

1.  Section  of  Horizontal  Strata     .         .         .         .         .         ...         .  7 

2.  Section  across  an  Anticline  .........  9 

3.  Section  across  Normal  Anticlines  and  Synclines  10 

4.  Section  across  Anticlines  and  Synclines  with  Inclined  Axes       .         .  10 

5.  Section  across  Faulted  or  Dislocated  Strata       .         .         .         .         .  n 

6.  Section  across  Unconformable  Strata        .         .         .         .         .         .41 

7.  Section  across  a  series  of  Alluvial  Terraces        .         .         .         .         .51 

8.  Section  and  Bird's-eye  View  of  Colorado  Plateau  (Powell)          .         .  54 

9.  Diagrammatic  Section  across  Colorado  Plateau          .         .         .         .58 

10.  Diagrammatic  Section  showing  Stages  of  Erosion  by  a  River  cutting 

through  Horizontal  Strata  (after  Captain  Dutton) ....  62 

11.  Section  across  Suderoe  (Far6e  Islands)  on  a  true  scale        ...  69 

12.  Map  of  an  Island  composed  of  Dome-shaped  Strata  ....  74 

13.  Section  through  the  Island  shown  in  Fig.  12     .         .         .  -74 

14.  Section  of  River-valley     .........  75 

15.  Enlarged  section  of  a  portion  of  the  Island  shown  in  Fig.  12    .         .  77 

16.  Diagram  Map  of  Plateau  of  Erosion 78 

17.  Section  across  reduced  Plateau  of  Erosion          .....  79 

18.  Longitudinal  Section  of  River  Course       .         .         ...         .80 

19.  Section  of  Escarpments  and  Outliers 84 

20.  Section  across  the  Wealdean  Area  (Ramsay) 84 

21.  Section  across  Permian  Volcanic  Basin,  Ayrshire       ....  86 

22.  Synclinal  Hills  and  Anticlinal  Valleys 87 

23.  Escarpment  Hills  and  Synclinal  Hill 88 

24.  Section  across  West  Lomond  Hill  and  the  Ochils      ....  88 

25.  Synclinal  Valley,  West  of  Green  River  (Powell)         ....  89 

26.  Anticlinal  Ridge,  Green  River  Plains  (Powell)  .....  90 

27.  Isoclinal  Folds          ..........  93 

28.  Isoclinal  Folds          ..........  94 

29.  Isoclinal  Folds 94 

30.  Overfold  passing  into  Reversed  Fault,  or  Overthrust          ...  95 

xi 


xii  LIST  OF  ILLUSTRATIONS 

FIGURE  PAGE 

31.  Reversed  Fault         .........  95 

32.  Single  Thrust-plane          .........       95 

33.  Section  across  Coal-basin  of  Mons  (M.  Bertrand)      .         .         .         .96 

34.  Section  from  Quinaig  to  Head  of  Glenbeg  (Geol.  Survey)  .         .       97 

35.  Synclinal  Double-fold       .........       97 

36.  Anticlinal  Double-fold 98 

37.  Diagram  of  Mountain  Flexures •       99 

38.  Diagram  of  Anticlinal  Mountains      .......     105 

39.  Synclinal  Valley  shifting  toward  Anticlinal  Axis        ....     106 

40.  Section  across  the  Swiss  Alps  (A.  Heim) .no 

41.  Summit  of  Santis,  East  Side  (A.  Heim)     .         .         .         .         .         .     ill 

42.  Section  across  the  Schortenkopf,  Bavarian  Alps  (E.  Fraas)        .  in 

43.  Section  across  the  Kaisergebirge,  Eastern  Alps  (E.  Fraas)          .         .112 

44.  Section  across  the  Val  d'Uina  (Gumbel)    .         .         .         .         .         .112 

45.  Sichelkamm  of  Wallenstadt  (Heim)  ......     112 

46.  Section  across  the  Northern  Limestone  Alps  (E.  Fraas)     .         .         .     113 

47.  Section  across  the  Diablerets  (Rene vier)  .          .         .         .         .         .113 

48.  Section  across  Dent  de  Morcles  (Renevier)         .         .         .         .         .114 

49.  Inversion  and  Overthrust  in  the  Mountains  South  of  the  Lake  of 

Wallenstadt  (E.  Fraas,  after  A.  Heim)          .         .         .         ..114 

50.  Symmetrical  Flexures  of  the  Jura  Mountains    .         .         .         .         .115 

51.  Section  across  Western  part  of  the  Jura  Mountains  (P.  Choffat)         .     116 

52.  Section  across  part  of  the  Sandstone-zone  of  the  Middle  Carpathians 

(Vacek) 116 

53.  Section  across  part  of  the  Middle  Carpathians  (Vacek)      .         .         .117 

54.  Section   across  the  Appalachian    Ridges   of   Pennsylvania   (H.    D. 

Rogers)         .         .         .         .         . 118 

55.  Unsymmetrical  Folds,  giving  rise  to  Escarpments  and  Ridges  .         .120 

56.  Structure  of  the  Ardennes  (after  Cornet  and  Briart)  ....     126 

57.  Diagrammatic  Section  across  a  Plateau  of  Erosion    .         .         .         .129 

58.  Section  across  portion  of  Southern  Uplands,  showing  Old  Red  Sand- 

stone resting  upon  Plain  of  Erosion      .         .         .         .         .         .136 

59.  Section  from  Glen  Lyon  to  Carn  Chois  (Geol.  Survey)       .         .         .     146 

60.  Section  of  Normal  Fault .         .         .         .         ,         .         .         .         .153 

61.  Normal  Fault,  with  High  Ground  on  Downthrow  Side     .         .         .     155 

62.  Normal  Fault,  with  High  Ground  on  Upcast  Side    ....     156 

63.  Faults  in  Queantoweep  Valley,  Grand  Canon  District  (Dutton)         .     158 

64.  Ranges  of  the  Great  Basin  (Hinman,  after  Gilbert :  length  of  section, 

120  miles) 159 

65.  Section  from  the  Mediterranean  across  the  Mountains  of  Palestine  to 

the  Mountains  of  Moab  (after  M.  Blanckenhorn)  ....     161 

66.  Section  across  the  Vosges  and  the  Black  Forest  (after  Penck)    .         .     164 


LIST  OF  ILLUSTRATIONS  xiii 

FIGURE  PAGE 

67.  Section  of  Coal-measures  near  Cambusnethan,  Lanarkshire,  on  a  true 

scale    .         .         .         .         .         ...'.'.         .         .         .  166 

68.  Section  on  a  true  scale  across  "  Tynedale  Fault,"  Newcastle  Coal-field  168 

69.  Section  across  Great  Fault  bounding  the  Highlands  near  Birnam, 

Perthshire 169 

70.  Section  across  Great  Fault  bounding  the  Southern  Uplands       .         .  170 

71.  Diagram  Section  across  Horstgebirge        .         .         .         .         .         .170 

72.  Mountain  of  Granite    .     .    .     »         .         .         .         .         .         .         .  175 

73.  Plain  of  Granite  overlooked  by  Mountains  of  Schists,  etc.         .         .  176 

74.  Diagrammatic  Section  of  a  Laccolith  showing  Dome-shaped  Eleva- 

tion of  Surface  above  the  Intrusive  Rock  (after  G.  K.  Gilbert)      .  177 

75.  View  of  Necks — Cores  of  old  Volcanoes  (Powell)      .         .         f:~    .  188 

76.  Section  of  Highly  Denuded  Volcano,  Minto  Hill,  Roxburgshire       .  189 

77.  Diagrammatic  Section  across  the  Valley  of  the  Tay,  near  Dundee      .  190 

78.  View  of  Mesa  Verde  and  the  Sierra  el  Late,  Colorado  (Hayden's  Re- 

port for  1875)        .         .         .         .         .         .         .         .         .         .  203 

79.  Wind  Erosion  :    Table-Mountains,  etc.,  of  the  Sahara  (Mission   de 

Chadames)  .         .         .         .         .         .         .    •    .         .         .         .  254 

80.  Wind   Erosion  :   Harder  Beds  amongst  inclined  Cretaceous  Strata, 

Libyan  Desert  (J.  Walther) 254 

81.  Wind  Erosion  :  Stages  in  the  Erosion  and  Reduction  of  a  Table- 

mountain  (J.  Walther)          ........  255 

82.  Manganese  Concretions  weathered  out  of  Sandstone,  Arabah  Mount- 

ains, Sinai  Peninsula  (J.  Walther) 256 

83.  Formation  of  Sand-dunes          ........  259 

84.  Advance  of  Sand-dunes 259 

85.  Longitudinal  Sections  of  Lake-basins  on  a  true  scale         .         .         .  293 

86.  Sea-cliff  cut  in  Horizontal  Strata 319 

87.  Sea-cliff  cut  in  Strata  dipping  Inland 320 

88.  Sea-cliff  cut  in  Strata  dipping  Seaward      ......  320 

89.  Sea-cliff  cut  in  Beds  dipping  Seaward 323 

FULL-PAGE  PLATES 

Plate  I.  Joints  in  Granite,  Glen  Eunach,  Cairngorm  (from  a  photograph 

by  W.  E.  Carnegie  Dickson) to  face  200 

Plate  II.  Weathering  of  Joints  in  Granite,  Cairngorm  Mountains  (from  a 

photograph  by  W.  E.  Carnegie  Dickson)  .  .  .  to  face  202 


EARTH  SCULPTURE 


CHAPTER   I 
INTRODUCTORY 

EARLY  VIEWS   AS  TO   ORIGIN   OF   SURFACE-FEATURES — ROCKS  AND 

ROCK-STRUCTURES ARCHITECTURE    OF    THE     EARTH'S     CRUST 

GENERAL    EVIDENCE    OF    ROCK-REMOVAL. 

WHEN  geologists  began  to  inquire  into  the  origin 
of  surface-features,  they  were  at  first  led  to 
believe  that  the  more  striking  and  prominent  of  these 
had  come  into  existence  under  the  operation  of  forces 
which  had  long  ago  ceased  to  affect  the  earth's  crust 
to  any  marked  extent.  It  is  not  hard  to  understand 
how  this  conception  arose.  The  earlier  observers 
could  not  fail  to  be  impressed  by  the  evidence  of 
former  crustal  disturbances  which  almost  everywhere 
stared  them  in  the  face.  Here  they  saw  mountains 
built  up  of  strangely  fractured,  contorted,  and  jum- 
bled rock-masses ;  there,  again,  they  encountered  the 
relics  of  vast  volcanic  eruptions  in  regions  now  practi- 
cally free  from  earth-throes  of  any  kind.  In  one  place 
ancient  land-surfaces  were  seen  intercalated  at  inter- 


2  EARTH  SCULPTURE 

vals  among  great  successions  of  marine  strata  ;  in 
other  places,  limestones,  evidently  of  oceanic  origin, 
were  found  entering  into  the  framework  of  lofty 
mountains  far  removed  from  any  sea.  It  was  these 
and  similar  striking  contrasts  between  the  present 
and  the  past  which  doubtless  induced  the  belief  that 
the  earth's  crust,  after  having  passed  through  many 
revolutions  more  or  less  catastrophic  in  character,  had 
at  last  become  approximately  stable — the  occasional 
earthquakes  and  volcanic  disturbances  of  recent  times 
being  looked  upon  as  only  the  final  manifestations  of 
those  forces  which  in  earlier  ages  had  been  mainly 
instrumental  in  producing  the  varied  configuration  of 
the  land.  Mountains  and  valleys  belonged  to  earth's 
Sturm  und  Drang  period.  That  wild  time  had 
passed  away,  and  now  old  age,  with  its  lethargy  and 
repose,  had  supervened.  The  tumultuous  accumula- 
tions of  stony  clay,  blocks  and  boulders,  gravel  and 
sand  that  overspread  extensive  areas  in  temperate 
latitudes  were  believed  to  be  the  relics  of  the  last 
great  catastrophe  which  had  affected  the  earth's  sur- 
face. Some  notable  disturbance  of  the  crust,  it  was 
thought,  had  caused  the  waters  of  northern  seas  to 
rush  in  devastating  waves  across  the  land.  When 
these  diluvial  waters  finally  retired,  then  the  modern 
era  began — an  era  characterised  by  the  more  equable 
operation  of  nature's  forces. 

But  with  increased  knowledge  these  views  gradu- 
ally became  modified.  Eventually,  it  was  recognised 
that  no  hard-and-fast  line  separates  past  and  present. 


INTRODUCTORY  3 

The  belief  in  world-wide,  or  nearly  world-wide,  cata- 
strophes disappeared.  Geologists  came  to  see  that  the 
fashioning  of  the  earth's  surface  had  been  going  on 
for  a  long  time,  and  is  still  in  progress.  The  law  of 
evolution,  they  have  found,  holds  true  for  the  crust 
of  the  globe  just  as  it  does  for  the  myriad  tribes  of 
plants  and  animals  that  clothe  and  people  it.  It  is  no 
longer  doubted  that  the  existing  configuration  of  the 
land  has  resulted  from  the  action  of  forces  that  are 
still  in  operation,  and  by  observation  and  reasoning 
the  history  of  the  various  phases  in  the  evolution  of 
surface-features  can  be  unfolded.  No  doubt  the 
evidence  is  sometimes  hard  to  read  in  all  its  details, 
but  its  general  bearing  can  be  readily  apprehended. 
The  salient  facts,  the  principal  data,  are  conspicuous 
enough,  and  the  mode  of  their  interpretation  is  in  a 
manner  self-evident. 

In  setting  out  upon  our  present  inquiry,  however, 
it  is  obvious  that  we  ought,  in  the  first  place,  to  know 
something  about  rocks  and  the  mode  of  their  arrange- 
ment. We  must  make  some  acquaintance  with  the 
composition  and  the  structure  or  architecture  of  the 
earth's  crust  before  we  can  form  any  reasonable  con- 
clusion as  to  the  origin  of  its  surface-features.  Now, 
so  far  as  that  crust  is  accessible  to  observation,  it  is 
found  to  be  built  up  of  two  kinds  of  rock,  one  set  be- 
ing of  igneous  origin,  while  the  other  appears  to  con- 
sist mainly  of  the  products  of  water  action.  These 
last  are  typically  represented  by  such  rocks  as  con- 
glomerate, sandstone,  and  shale,  which  are  only  more 


4  EARTH  SCULPTURE 

or  less  ancient  sediments,  formed  and  accumulated  in 
the  same  way  as  the  gravel,  sand,  and  mud  of  existing 
rivers,  lakes,  and  seas.  Another  common  rock  of 
aqueous  origin  is  limestone,  of  which  there  are  count- 
less varieties — some  formed  in  lakes,  like  the  shell- 
marls  of  our  own  day  ;  others  representing  the 
calcareous  ooze  arid  coral-reefs  of  ancient  seas  ;  while 
yet  others  are  obviously  chemical  precipitates  from 
water  surcharged  with  carbonate  of  lime.  Now  and 
again,  also,  we  meet  with  rocks  of  terrestrial  origin, 
such,  for  example,  as  many  beds  and  seams  of  peat, 
lignite,  and  coal,  which  are  simply  the  vegetable  debris 
of  old  land-surfaces.  To  these  land-formed  beds  we 
may  add  certain  sandstones  of  wind-blown  origin — 
indurated  sand-dunes,  in  short. 

The  igneous  rocks  consist  partly  of  lavas  and  frag- 
mental  materials  which  have  been  ejected  at  the  sur- 
face, as  in  modern  volcanoes,  and  partly  of  formerly 
molten  masses  which  have  cooled  and  consolidated 
below  ground.  The  former,  therefore,  are  spoken  of 
as  volcanic,  the  latter  as  plutonic  or  hypogene  rocks. 
As  it  is  useful  to  have  some  general  name  for  the 
rocks  which  owe  their  origin  to  the  action  of  epigene 
agents  (z.  e.,  the  atmosphere,  terrestrial  water,  ice,  the 
sea,  and  life),  we  may  term  these  derivative,  since  they 
have  been  built  up  chiefly  out  of  the  relics  of  pre-ex- 
isting rocks  and  the  cUbrisvi  plants  and  animals.  By- 
and-by  we  shall  learn  that  igneous  and  derivative 
rocks  have  in  certain  regions  been  subjected  to  many 
remarkable  changes,  and  are  in  consequence  so 


IN  TROD  UCTOR  Y  5 

altered  that  it  is  often  hard  to  detect  their  original 
character.  These  altered  masses  form  what  are  called 
the  metamorphic  rocks.  They  are  typically  repre- 
sented by  such  rocks  as  gneiss,  mica-schist,  clay-slate, 
etc. 

The  derivative  rocks,  with  which  in  many  regions 
igneous  rocks  are  associated,  occupy  by  far  the  larger 
portion  of  the  land-surface,  entering  abundantly  into 
the  composition  of  low  grounds  and  mountains  alike. 
Most  of  these  derivatives  are  sedimentary  accumula- 
tions, and  very  many  are  charged  with  the  remains  of 
animals  and  plants.  By  noting  the  order  in  which 
such  stratified  deposits  occur,  and  by  comparing  and 
correlating  their  fossils,  geologists  have  been  able  to 
group  them  into  a  series  of  successive  systems,  the 
oldest  being  that  which  occurs  at  the  bottom  of  the 
series.1  The  united  thickness  of  the  several  systems 
probably  exceeds  twenty  miles,  but  it  must  not  be 
supposed  that  all  these  occur  together  in  any  one 
region.  Many  broad  acres  of  the  earth's  surface  are 
occupied  by  the  rocks  belonging  to  one  system  only. 
In  other  countries  two  or  more  systems  may  be  present. 
Again,  each  individual  system  is  of  very  variable 
thickness — swelling  out  here,  thinning  off  there  :  in 
some  lands  being  represented  by  strata  many  thou- 
sands of  feet  in  thickness,  in  others  dwindling  down 
to  a  few  yards.  In  short,  we  may  picture  to  ourselves 
each  system  as  consisting  of  a  series  of  larger  and 
smaller  lenticular  sheets,  irregularly  distributed  over 

1  See  Appendix  for  Table  of  Geological  Systems. 


6  EARTH  SCULPTURE 

the  earth's  surface.  The  various  systems  thus  fre- 
quently overlap,  the  younger  stealing  over  the  surface 
of  the  older  so  as  often  to  bury  these  out  of  sight. 

The  metamorphic  rocks  do  not  appear  at  the  sur- 
face over  such  extensive  areas  as  those  just  referred 
to.  Nevertheless,  they  are  widely  distributed,  and 
now  and  again  overspread  continuously  vast  regions. 
The  enormous  tract  that  extends  from  the  Great  Lakes 
of  North  America  to  the  shores  of  the  Arctic  Ocean 
is  almost  entirely  occupied  by  them.  Another  im- 
mense area  of  crystalline  schistose  rocks  is  met  with 
in  Brazil.  The  Highlands  of  Scotland,  the  Scandi- 
navian Peninsula,  and  .North  Finland  are  in  like  man- 
ner largely  composed  of  them,  and  the  same  is  the 
case  with  many  parts  of  Africa,  Asia,  and  Australia. 
It  is  further  noteworthy  that  similar  rocks  form  the 
backbones  of  most  of  the  great  mountain  chains  of 
the  globe.  As  already  indicated,  metamorphic  rocks 
are  of  various  origin,  some  of  them  being  primarily 
of  igneous  and  others  of  aqueous  formation.  Those 
which  form  the  nuclei  of  the  youngest  mountain 
chains  are  sometimes  of  relatively  recent  age,  while 
those  occupying  such  broad  tracts  as  Brazil,  the 
Canadian  uplands,  etc.,  are  of  vast  antiquity.  Crys- 
talline schistose  rocks,  with  associated  granites  and 
other  igneous  rocks,  seem  everywhere  to  underlie 
the  sedimentary  fossiliferous  formations.  Very  often 
the  latter  are  separated  by  a  broadly  marked  line  of 
demarcation  from  the  schists,  granites,  etc.,  upon  which 
they  repose.  In  other  cases  the  sedimentary  rocks 


IN  TROD  UCTOR  Y  7 

become  gradually  altered  as  they  are  traced  down- 
wards, until  eventually  they  themselves  assume  the 
aspect  of  crystalline  schists,  penetrated  here  and  there 
by  granitoid  igneous  rocks. 

The  origin  of  those  ancient  crystalline  schists  has 
been  much  discussed,  but  does  not  concern  us  here. 
Some  geologists  have  maintained  that  the  rocks  in 
question  represent  the  original  cooled  crust  of  the 
globe,  while  the  majority  consider  them  to  be  all 
metamorphic.  It  is  enough  for  our  present  pur- 
pose to  know  that  a  pavement  of  such  rocks  appears 
everywhere  to  underlie  the  sedimentary  fossiliferous 
formations. 


FIG.  i.     SECTION  OF  HORIZONTAL  STRATA. 

The  upper  continuous  line,  A  -J9,  =  surface  of  ground  ;  the  lower  continuous  line,  C-D,  =  sea- 
level;  /,  limestone,;  JE,  sandstones  and  shales. 

The  great  bulk  of  the  derivative  rocks  being  of 
sedimentary  origin,  it  is  obvious  that  they  must  have 
been  at  the  time  of  their  formation  spread  out  in  ap- 
proximately horizontal  layers  upon  the  beds  of  ancient 
lakes  and  seas.  This  we  are  justified  in  believing 
by  what  we  know  of  the  accumulation  of  similar 
sediments  in  our  own  day.  The  wide  flats  of  our  river- 
valleys,  the  broad  plains  that  occupy  the  sites  of  silted- 
up  lakes,  the  extensive  deltas  of  such  rivers  as  the 
Nile,  the  Po,  the  Amazon,  the  Mississippi,  the  narrow 


8  EARTH  SCULPTURE 

or  wide  belts  of  low-lying  land  which  within  a  recent 
period  have  been  gained  from  the  sea,  are  all  made  up 
of  various  kinds  of  sediment  arranged  in  gently  in- 
clined or  approximately  horizontal  layers.  Now,  over 
considerable  areas  of  the  earth's  surface  the  derivative 
rocks  show  the  same  horizontal  arrangement,  a  struct- 
ure which  is  obviously  original.  And  this  is  frequently 
the  case  with  younger  and  older  sedimentary  strata 
alike.  Here,  for  example  (Fig.  i),  is  a  section  across 
a  country,  the  superficial  rock-masses  of  which  are 
horizontally  arranged. 

The  upper  line  of  the  section  (A-B)  represents,  of 
course,  the  surface  of  the  ground,  while  the  lower  we 
shall  take  to  be  the  level  of  the  sea.  The  section 
thus  shows  the  geological  structure  or  arrangement 
of  the  rocks  from  the  surface  down  to  the  level  of  the 
sea.  The  strata  represented  consist  of  a  great  series 
of  sandstones  and  shales  with  one  prominent  bed  of 
limestone  (/)  at  the  top.  In  this  case  we  cannot 
doubt  that  the  horizontal  bedding  is  original — that 
the  strata  were  accumulated  one  above  the  other  in 
the  same  order  as  we  see  them. 

Although  such  horizontal  arrangements  are  of  com- 
mon enough  occurrence,  and  now  and  again  charac- 
terise the  sedimentary  systems  over  wide  areas,  yet, 
as  a  general  rule,  strata  tend  to  be  inclined.  In  many 
regions  the  inclination,  or  dip,  as  it  is  termed,  is  some- 
times very  high — not  seldom  indeed  the  beds  are  seen 
standing  on  end,  like  rows  of  books  in  a  library.  This 
last  appearance  of  extreme  disturbance  is  not  confined 


INTRODUCTORY  9 

to  the  strata  of  any  system  ;  nevertheless,  it  is  more 
characteristic  of  the  older  than  the  younger  systems. 
In  the  sequel  we  shall  have  to  study  these  and  other 
rock-structures  more  particularly,  but  for  the  present 
we  need  not  do  more  than  make  some  general 
acquaintance  with  them. 

A  very  common  arrangement  is  shown  in  the  next 
diagram  (Fig.  2).  Here  the  strata  are  arranged  in 
the  form  of  a  truncated  arch,  or  anticline.  At  X  the 


A.-"' 


C  V 

FIG.  2.    SECTION  ACROSS  AN  ANTICLINE. 

The  upper  continuous  line,  A-B,  =  surface  of  ground  ;  the  lower  continuous  line,  C-D,  =  sea- 
level;  X-Y,  =  vertical  axis. 

beds  are  approximately  horizontal,  but  from  this  point 
they  dip  on  the  right  towards  B,  and  on  the  left  in 
the  direction  of  A.  Note  further  that  the  angle  of 
inclination  is  the  same  on  each  side  of  the  anticline ; 
in  other  words,  the  anticlinal  axis  (X-Y)  is  vertical. 
From  A  to  B  the  distance  we  shall  suppose  is  six 
miles. 

The  succeeding  section  (Fig.  3)  we  shall  take  to 
be  of  equal  length.  Here  we  have  a  succession  of 
anticlines,  or  saddle-backs,  separated  one  from  another 
by  troughs,  or  sync 'lines,  as  they  are  termed.  In  other 


10 


EARTH  SCULPTURE 


words,  the  strata  are  undulating.  From  these  sections 
we  learn  that  folds  or  undulations  vary  considerably 
in  width.  -In  the  region  represented  by  Fig.  2  we 
have  an  area  six  miles  in  breadth,  consisting  of  a 
thick  series  of  strata  disposed  in  the  form  of  one  sin- 
gle arch  or  anticline  ;  while  in  Fig.  3,  representing 


*  «  *  *  L    D 

FIG.  3.    SECTION  ACROSS  SYMMETRICAL  ANTICLINES  AND  SYNCLINES. 

Upper  continuous  line,  A-B,  =  surface  of  ground  ;    lower  continuous  line,  C-A  =  sea-level ; 
a  a,  anticlines ;  j  j,  synclines  ;  a  x,  s  x,  axes  of  folds. 

an  equal  area,  the  strata  are  folded  into  a  series  of 
several  anticlines  and  synclines.  In  both  regions  the 
anticlines  are  symmetrical ;  that  is  to  say,  their  axes 
(a  x,  s  x)  are  vertical. 

But  folds  or  undulations  may  follow  each  other 
much  more  rapidly  than  is  shown  in  the  preceding 
section.  In  countries  built  up  of  steeply  inclined 


FIG.  4.     SECTION  ACROSS  UNSYMMETRICAL  ANTICLINES  AND  SYNCLINES. 

Upper  continuous  line,  A-B^  =  surface  of  ground  ;   lower  continuous  line,  C-Z>,  =  sea-level; 
a  .r,  s  x,  axes  of  folds. 

rocks,  the  undulations  of  the  strata  are  more  abrupt, 
and  the  axes  of  the  folds  are  frequently  inclined.     In 


IN  TROD  UCTOR  V 


ii 


Fig.  4,  for  example,  most  of  the  anticlines  and  syn- 
clines  lean  over  to  one  side,  and  this  to  such  a  de- 
gree, that  here  and  there  upper  beds  are  doubled  un- 
der older  beds  of  the  same  series  of  strata ;  in  other 
words,  the  order  of  succession  appears  to  be  inverted. 
From  the  fact  that  strata  are  generally  inclined 
from  the  horizontal,  and  frequently  curved  and  folded, 
it  is  obvious  that  they  have  been  subjected  to  the 
action  of  some  great  disturbing  force,  for  folding  and 


FIG.  5.    SECTION  ACROSS  FAULTY  OR  DISLOCATED  STRATA. 

/,  normal  fault,  inclined  in  the  direction  of  downthrow. 

contortion  may  affect  masses  of  strata  many  thousands 
of  feet  in  thickness.  Another  evident  mark  of  dis- 
turbance is  furnished  by  the  presence  of  dislocations, 
or  faults,  as  they  are  technically  termed,  along  the 
line  of  which  the  rocks  have  been  shifted  for,  it  may 
be,  hundreds  and  sometimes  even  for  thousands  of 
feet.  One  of  the  simplest  kind  of  faults  is  shown  in 


12  EARTH  SCULPTURE 

the  preceding  illustration  (Fig.  5).  Here,  as  in 
preceding  figures,  the  upper  line  (A-H)  represents 
the  surface  of  the  ground.  At  f  the  strata  are  tra- 
versed by  a  fault,  which  has  caused  a  vertical  displace- 
ment of  the  beds  to  the  extent  of,  say,  500  feet,  for  it 
is  obvious  that  the  coal  and  fireclay  (8,  9),  and  the 
strata  amongst  which  they  lie  on  the  left-hand  side, 
were  formerly  continuous  with  the  corresponding 
beds  on  the  other  side  of  the  fault. 

From  the  facts  now  briefly  set  forth  we  may  draw 
certain  conclusions.  In  the  first  place,  the  extensive 
geographical  range  of  the  derivative  rocks,  most  of 
which  are  of  marine  origin,  must  convince  us  that  the 
greater  portion  of  our  continental  areas  has  been  un- 
der water.  It  is  not  to  be  understood,  however,  that 
all  the  land-surfaces  occupied  by  sedimentary  strata 
have  been  submerged  at  one  and  the  same  time.  On 
the  contrary,  the  several  geological  systems  have  been 
accumulated  at  widely  different  periods.  This  is  a 
point,  however,  to  which  we  shall  return  :  for  the 
present,  we  need  only  keep  in  view  the  prominent 
fact  that  the  existing  land-surfaces  of  the  globe  are 
composed  most  frequently  of  marine  strata.  There 
are  apparently  only  two  ways  in  which  this  phenom- 
enon can  be  accounted  for,  and  these  explanations 
come  to  much  the  same  thing.  Either  the  general 
level  of  the  ocean  has  fallen,  or  wide  areas  of  the  sea- 
floor  have  been  pushed  up  from  below  and  converted 
into  dry  land.  Both  changes  appear  to  have  taken 
place.  The  bed  of  the  sea  has  sunk  from  time  to 


INTRODUCTORY  13 

time  to  greater  and  greater  depths,  and  has  thus 
tended  to  draw  the  water  away  from  the  surface  of 
what  are  now  continental  areas.  But  if  the  earth's 
crust  under  the  ocean  has  subsided,  it  has  also  been 
elevated  within  what  are  now  dry  lands  again  and 
again.  The  folds  and  corrugations  of  the  strata,  and 
the  numerous  dislocations  by  which  rocks  of  all  kinds 
are  traversed,  clearly  demonstrate  that  movements  of 
the  solid  crust  have  taken  place.  Such  crustal  dis- 
turbances are  probably  in  chief  measure  due  to  the 
fact  that  the  earth  is  a  cooling  body.  As  the  solid 
crust  sinks  down  upon  the  cooling  and  contracting  nu- 
cleus, it  must  occupy  less  superficial  space.  Hence  its 
rocky  framework  becomes  subjected  to  enormous  tan- 
gential squeezing  and  compression  to  which  it  yields 
by  bending  and  folding,  by  fracture  and  displacement. 
Obviously,  then,  the  mysterious  subterranean  forces 
must  have  played  an  important  part  in  the  formation 
of  earth-features.  Disturbed  rocks  are  of  more  fre- 
quent occurrence  than  strata  which  have  retained 
their  original  horizontality.  It  is  no  wonder,  there- 
fore, that  for  a  long  time  the  general  configuration  of 
the  land  was  believed  to  have  been  impressed  upon  it 
by  plutonic  agency.  Indeed,  in  the  case  of  certain 
mountain  chains,  we  cannot  fail  to  see  that  the  larger 
features  of  such  regions  often  correspond  to  a  con- 
siderable extent  with  the  main  flexures  and  displace- 
ments of  the  underlying  rocks.  In  many  elevated 
tracts,  however,  composed  of  highly  disturbed  and 
contorted  strata,  no  such  coincidence  of  surface-feat- 


14  EARTH  SCULPTURE 

ure  and  underground  structure  can  be  traced.  The 
mountain  ridges  do  not  correspond  to  great  swellings 
of  the  crust ;  the  valleys  neither  lie  in  trough-shaped 
strata,  nor  do  they  coincide  with  gaping  fractures. 
Again,  many  considerable  mountains  are  built  up  of 
rocks  which  are  not  convoluted  at  all,  but  arranged  in 
horizontal  beds.  More  than  this,  many  plateaux  and 
even  lowlands  are  composed  of  as  highly  flexed  and  con- 
torted strata  as  are  to  be  met  with  in  any  mountainous 
country.  Evidently,  therefore,  crustal  movement  is  not 
the  only  factor  in  the  production  of  surface-features. 

The  sections  already  given  will  serve  to  illustrate 
the  general  fact  that  underground  structure  and  su- 
perficial configuration  do  not  necessarily  correspond. 
Thus  in  Fig.  i  we  have  a  series  of  pyramidal  mount- 
ains developed  in  horizontal  strata.  The  •  slope  of 
the  surface,  therefore,  frequently  bears  no  relation  to 
the  "lie"  of  the  beds  below.  This  is  further  illus- 
trated in  the  succeeding  figures,  where  we  find  de- 
pressions at  the  surface,  while  the  rocks  immediately 
underneath  show  an  anticlinal  arrangement ;  and,  con- 
versely, where  the  strata  are  trough-shaped  the  sur- 
face-feature is  not  a  depression  but  an  elevation. 

In  the  case  of  the  horizontal  strata  shown  in  Fig. 
i  we  have  no  difficulty  in  perceiving  that  the  present 
surface  is  not  that  of  original  deposition.  It  is  impos- 
sible that  sedimentary  deposits  could  have  been  piled 
up  in  the  shape  of  great  pyramids  :  obviously  the  beds 
were  formerly  continuous,  as  shown  by  the  dotted  lines. 
Clearly  some  "  monstrous  cantles  "  have  been  cut  out 


IN  TROD  UCTOR  Y  15 

and  removed.  And  the  same  is  necessarily  true  of 
the  folded  strata.  In  each  case  (Figs.  2,  3,  4)  masses 
of  strata  have  disappeared  ;  the  tops  or  backs  of  the 
anticlinal  arches  have  been  more  or  less  deeply  incised, 
and  the  material  carried  away.  In  subsequent  pages 
it  will  be  shown  that  the  thickness  of  rocks  thus  re- 
moved can  be  proved  to  amount  in  many  cases  to 
thousands  of  feet. 

Not  less  striking  is  the  evidence  of  rock-removal 
furnished  by  the  phenomena  of  faults.  At  the  sur- 
face there  may  be  no  inequality  of  level  corresponding 
to  that  seen  below  (see  Fig.  5).  Obviously,  how- 
ever, a  considerable  thickness  of  rock  has  vanished. 
Were  the  missing  continuations  of  the  strata  to  be 
replaced  upon  the  high  side  of  the  fault,  they  would 
occupy  the  space  contained  within  the  dotted  lines 
above  the  present  surface  A—B.  Such  dislocations 
often  interrupt  the  continuity  of  the  strata  in  our  coal- 
fields. In  such  regions  we  may  traverse  level  or 
gently  undulating  tracts,  and  be  quite  unconscious  of 
the  fact  that  geologically  we  have  several  times  leaped 
up  or  jumped  down  hundreds  of  feet  in  a  single  step. 
Nay,  some  rivers  flow  across  dislocations  by  which 
the  strata  have  been  shifted  up  or  down  for  thousands 
of  yards,  and  in  some  places  we  may  sit  upon  rocks 
which  are  geologically  more  than  a  thousand  fathoms 
below  or  above  those  on  which  we  rest  our  feet. 
Faults,  then,  afford  clear  evidence  of  the  wholesale 
removal  of  rocks  from  the  surface  of  the  land. 

Such  proofs  of  rock-removal  can  be  appreciated  by 


16  EARTH  SCULPTURE 

anyone,  and  will  come  frequently  before  us  in  the 
discussion  that  follows.  There  is  another  kind  of 
evidence,  however,  leading  to  the  same  general  con- 
clusion, which  may  be  briefly  touched  upon  at  this 
stage  of  our  inquiry.  In  this  and  other  countries 
there  are  enormous  masses  of  rock,  often  widely  ex- 
tended, which  have  cooled  and  consolidated  from  a 
state  of  igneous  fusion.  Some  of  these,  it  is  well 
known,  have  flowed  out  as  lavas  at  the  surface,  while 
others  were  never  so  erupted,  but  have  solidified 
at  greater  or  less  depths  below  ground.  Among  the 
latter  is  granite,  a  rock  believed  to  be  of  deep-seated 
origin.  Its  plutonic  character  is  evinced  not  less  by 
its  composition  and  structure  than  by  its  relation  to 
the  rock-masses  that  surround  it.  Every  mass  of 
granite,  then,  has  cooled  and  consolidated,  probably 
very  slowly,  and  certainly  at  a  less  or  greater  depth 
in  the  earth's  crust.  When  this  rock  is  met  with  over 
a  wide  area  at  the  actual  surface,  therefore, — forming, 
it  may  be,  great  mountains  or  rolling  and  broken  low- 
lands,— we  know  that  in  such  regions  thick  masses  of 
formerly  overlying  rocks  have  been  removed.  The 
granite  appears  at  the  surface  simply  because  the 
covering  of  rocks  underneath  which  it  cooled  and 
solidified  has  been  subsequently  carried  away. 

The  occurrence  at  the  surface  of  crystalline  schists 
and  other  metamorphic  rocks  has  a  similar  significance. 
Although  the  processes  by  which  rocks  become  so 
highly  altered  are  still  more  or  less  obscure,  yet  there 
can  be  no  doubt  that  the  metamorphism  had  taken 


INTRODUCTORY  17 

place   when    the    rocks   affected  were  more   or  less 
deeply  buried  in  the  crust. 

While  we  may  safely  infer,  from  the  general  phe- 
nomena of  geological  structure,  that  earth-movements 
have  shared  in  the  production  of  surface-features,  we 
must  be  convinced,  at  the  same  time,  that  some  other 
factor  has  aided  in  the  work  of  shaping  out  our  lands. 
Earth-movements  quite  account  for  the  folding  and 
fracturing  of  strata,  for  the  uplifting  of  great  mount- 
ain masses,  but  they  cannot  have  caused  the  general 
loss  which  these  masses  have  sustained.  We  may 
conceive  it  possible  that  subterranean  action  may 
now  and  again  have  resulted  in  wide-spread  shattering 
of  rocks  at  the  surface,  but  such  action  could  not  have 
caused  the  broken  material  to  disappear.  Further, 
when  we  bear  in  mind  that  the  thickness  of  rock 
removed  from  the  surface  of  the  land  is  sometimes  to 
be  measured  by  many  thousands  of  feet,  or  even  yards, 
we  see  at  once  that  subterranean  action  cannot  have 
been  directly  implicated  in  the  spoliation  of  the  land. 
How,  then,  have  anticlines  been  truncated  ?  What 
power  has  removed  the  strata  from  the  high  side  of  a 
fault  ?  What,  in  a  word,  has  produced  that  trunca- 
tion and  discontinuity  of  beds  which  is  so  common  a 
feature  of  derivative  rocks  all  the  world  over  ?  And 
how  shall  we  account  for  the  presence  at  the  sur- 
face of  deep-seated  plutonic  rocks  and  metamorphic 
masses  ?  When  we  have  satisfactorily  answered 
such  questions  we  shall  have  solved  the  problem  of 
the  origin  of  surface-features. 


CHAPTER   II 

AGENTS  OF  DENUDATION 

CHEMICAL    COMPOSITION     OF     ROCKS — EPICENE     AGENTS — INSOLA- 
TION   AND   DEFLATION CHEMICAL    AND    MECHANICAL  ACTION 

OF    RAIN ACTION    OF   FROST  ;    OF    PLANTS   AND  ANIMALS  ;    OF 

UNDERGROUND    WATER  ;     OF    BROOKS    AND    RIVERS RATE  OF 

DENUDATION — DENUDATION    AND    SEDIMENTATION    GO    HAND 
•IN    HAND. 

T^HE  present,  geologists  tell  us,  contains  the  key  to 
*  the  past.  If  we  wish  to  find  out  how  rocks  have 
been  removed,  and  what  has  since  become  of  them, 
we  must  observe  what  is  taking  place  under  the 
influence  of  existing  agents  of  change.  How,  then, 
are  rocks  being  affected  at  present  ?  We  do  not  pro- 
ceed far  in  our  investigation  before  we  discover  that 
they  are  everywhere  becoming  disintegrated.  In  one 
place  they  are  breaking  up  into  angular  fragments  ; 
in  another,  crumbling  down  into  grit,  sand,  or  clay. 
Brooks  and  rivers  and  the  waves  upon  our  coasts  are 
constantly  undermining  them  ;  everywhere,  in  short, 
rocks  are  being  assaulted  and  reduced.  But  in  order 
to  bring  this  fact  more  forcibly  before  the  reader,  it 
will  be  well  to  sketch,  as  briefly  as  may  be,  the  general 
character  of  the  warfare  which  is  being  waged  against 

18 


AGENTS  OF  DENUDATION  19 

rocks  over  all  the  land-surface,  and  to  note  the  various 
results  that  flow  from  this  incessant  energy  of  the 
epigene  or  superficial  agents  of  change. 

As  these  agents  are  often  associated  in  their  work, 
it  is  sometimes  hard,  or  even  impossible,  to  say  which 
has  played  the  most  effective  part  in  the  demolition 
of  rocks.  Nevertheless,  it  will  conduce  to  clearness 
if  we  endeavour  to  consider  the  operation  of  each  by 
itself,  so  far,  at  least,  as  that  is  possible.  Before 
doing  so,  however,  we  must  glance  for  a  moment  at 
the  general  characters  of  rocks.  We  have  already 
taken  note  of  the  fact  that  rocks  are  of  various  origin 
—igneous,  derivative,  and  metamorphic.  It  is  now 
necessary  to  consider  their  composition  and  structure, 
for,  according  as  these  differ,  rocks  are  variously 
affected  by  epigene  agents,  some  yielding  rapidly, 
others  being  more  resistant.  We  need  not  go  into 
detail.  Their  composition  and  structure  may  be  de- 
scribed in  the  most  general  terms.  For  our  purpose 
it  will  suffice  to  group  them  roughly  under  these  four 
heads  :  Felspathic,  Argillaceous,  Silicious,  and  Cal- 
careous rocks.  This  is  very  far  from  being  an 
exhaustive  classification,  but  under  these  groups  may 
be  included  all  the  rocks  that  enter  most  largely  into 
the  formation  of  the  earth's  crust. 

i.  Felspathic  Rocks.  These  rocks  contain  as  their 
dominant  constituent  the  mineral,  or,  rather,  the 
family  of  minerals,  known  under  the  name  of  felspar. 
The  group  includes  nearly  all  the  igneous  and  most 
of  the  metamorphic  rocks.  The  derivative  rocks  that 


20  EARTH  SCULPTURE 

come  under  the  same  head  are  of  relatively  small 
importance.  The  minerals  entering  most  abundantly 
into  the  composition  of  the  felspathic  rocks  are  the 
felspars  (aluminous  silicates  of  potash,  soda,  and  lime), 
various  ferro  -  magnesian  silicates,  such  as  mica,  py- 
roxene, hornblende,  and  olivine  (aluminous  silicates 
of  magnesia,  lime,  iron-oxides,  etc.),  and  quartz  (silica, 
silicic  acid).  The  crystalline  igneous  rocks  occur 
either  in  more  or  less  regular  beds  (lavas),  interstrati- 
fied  with  derivative  rocks,  or  they  penetrate  these  in 
the  form  of  irregular  veins,  dykes,  sheets,  or  large 
amorphous  masses.  The  lava-form  rocks  are  often 
associated  with  beds  of  volcanic  debris  (tuff,  etc.). 
Some  igneous  rocks  are  smoothly  compact  in  texture, 
such  as  obsidian  and  pitchstone,  which  are  simply 
varieties  of  volcanic  glass ;  others,  such  as  basalt, 
consist-partly  of  glass  and  partly  of  crystalline  in- 
gredients, and  vary  in  texture  from  compact  to  coarse- 
grained ;  yet  others  are  built  up  wholly  of  crystalline 
substances,  and  may  be  fine-grained  or  very  coarsely 
granular,  as  granite.  The  crystalline  schists  are 
equally  variable  as  regards  texture.  They  differ, 
however,  from  the  igneous  rocks  in  structure.  While 
the  latter  are  confusedly  crystalline,  the  schists  show 
a  kind  of  streaky  structure  or  pseudo-lamination, 
their  constituent  minerals  being  arranged  in  rudely 
alternate  lenticular  layers. 

Igneous  rocks  and  schists  are  traversed  by  cracks 
and  fissures  which  usually  ramify  irregularly  in  all 
directions.  In  many  bedded  igneous  rocks  (lavas), 


AGENTS  OF  DENUDATION  21 

however,  these  cracks,  or  "joints,"  as  they  are  termed, 
are  somewhat  more  regular,  being,  as  a  rule,  disposed 
at  approximately  right  angles  to  the  planes  of  bed- 
ding. In  certain  fine-grained  rocks,  such  as  basalt, 
the  jointing  is  often  very  regular,  giving  rise  to  a 
prismatic  columnar  structure,  as  in  the  basalts  of  Staffa 
and  the  Giant's  Causeway.  The  main  fact,  however, 
with  which  we  are  at  present  concerned  is  simply  this  : 
that  all  crystalline,  igneous,  and  schistose  rocks  are 
traversed  by  cracks  and  fissures  of  one  sort  or  another. 
It  is  further  to  be  noted  that  these  rocks,  in  common 
with  rocks  of  all  kinds,  are  more  or  less  porous,  and 
therefore  liable  to  be  permeated,  however  slowly,  by 
percolating  water. 

2.  Argillaceous  Rocks.     These  rocks  are  composed 
chiefly  of  clay,  but  other  ingredients  are  usually  pre- 
sent.    Some   are  soft,   such   as  ordinary    brick-clay ; 
others  are  of  firmer  consistency,  and  frequently  show 
a   fine    fissile  structure,  as   in  common   argillaceous 
shale  ;  yet    others    are    hard,   tough   rocks,   some   of 
which  are  capable  of  being  cleaved  into  thin  plates, 
as  roofing-slate. 

3.  Silicious  Rocks.     These  might  be  described  in 
general   terms  as  gravel-and-sand  rocks.     The  most 
abundant  and  widely  distributed  rocks  of  this  class 
are  the  sandstones — composed  generally  of  grains  of 
quartz  (silica)    cemented   together   by  carbonate    of 
lime,  by  iron-oxide,  or  other  substance.     Cementing 
material,  however,  is  not  always  present,  some  sand- 
stones having  been  solidified  by  pressure  alone.     The 


22  EARTH  SCULPTURE 

gravel-rocks,  or  conglomerates,  usually  consist  of 
rounded  fragments  of  quartz  or  some  hard  silicious 
rock.  But  to  this  there  are  exceptions,  the  stones  in 
some  conglomerates  consisting  of  calcareous  or  of 
felspathic  rocks  or  of  a  mixture  of  many  different 
kinds.  A  silicious  sandstone  which  has  been  more  or 
less  metamorphosed  is  termed  quartz-rock. 

4.  Calcareous  Rocks.  Under  this  head  are  grouped 
limestones  of  every  kind.  They  vary  in  character 
from  soft  earthy  marls  and  chalks  to  hard,  granular, 
crystalline  limestones  and  saccharoid  marbles.  Some 
are  nearly  pure  carbonate  of  lime  ;  others  contain 
larger  or  smaller  percentages  of  quartz,  clay,  iron- 
oxide,  and  other  impurities. 

The  Argillaceous,  Silicious,  and  Calcareous  groups 
comprise  the  great  bulk  of  the  derivative  rocks  as 
well  as  a  few  metamorphic  rocks.  They  are  all  origin- 
ally of  aqueous  or  sedimentary  origin,  and  generally 
occur,  therefore,  in  beds  or  strata.  Like  the  igneous 
rocks,  they  are  more  or  less  porous,  although  some — 
especially  the  clay-rocks — are  much  less  permeable 
than  others.  In  addition  to  the  planes  of  lamination 
and  stratification,  which  characterise  most  derivative 
rocks,  there  are  other  natural  division-planes  or  joints 
which  cut  across  the  strata  in  directions  more  or  less 
perpendicular  to  the  bedding.  More  irregular  usually 
are  the  joints  which  intersect  hard  slates  and  quartz- 
rock,  these  being  divided  generally  much  in  the  same 
way  as  schists  and  amorphous  masses  of  crystalline 
igneous  rock. 


AGENTS  OF  DENUDATION  23 

There  are  not  a  few  kinds  of  rock  other  than  those 
now  referred  to,  but  they  may  be  neglected  as,  from 
our  present  point  of  view,  of  relatively  little  import- 
ance. Amongst  them  are  rock-salt,  gypsum,  coal 
and  lignite,  ironstones,  and  other  ores.  All  these, 
doubtless,  are  very  notable  and  valuable,  but  they  are 
neither  so  abundant  nor  so  widely  distributed  as  the 
above-described  groups  ;  in  short,  they  occupy  a  very 
subordinate  place  in  the  architecture  of  the  earth's 
crust. 

We  have  now  to  consider  how  the  superficial  or 
epigene  agents  attack  and  reduce  rocks.  And  first, 
we  may  note  that  rocks  at  the  surface  are  everywhere 
subject  to  changes  of  temperature — warmed  by  day 
and  during  summer,  cooled  at  night  and  during  win- 
ter. Thus  they  alternately  expand  and  contract,  and 
this  tends  to  disintegration,  for  the  materials  of  which 
they  are  composed  often  yield  unequally  to  strain  or 
tension.  This  is  particularly  the  case  with  many  crys- 
talline felspathic  rocks,  such  as  coarse-grained  granite, 
gneiss,  and  mica-schist — built  up,  as  these  are,  of  min- 
erals that  differ  in  colour,  density,  and  expansibility. 
Even  when  a  rock  is  homogeneous  in  composition,  it 
is  obvious  that  the  heating  and  cooling  of  the  surface 
must  give  rise  to  strain  and  tension.  In  countries 
where  there  is  no  great  diurnal  range  of  temperature, 
as  in  our  own  latitudes,  any  rock-changes  due  to  this 
cause  alone  are  hardly  noticeable,  since  they  are 
masked  or  obscured  by  the  action  of  other  and  more 
potent  agents.  But  in  the  rocky  deserts  of  tropical 


24  EARTH  SCULPTURE 

and  sub-tropical  regions,  bare  of  verdure  and  practi- 
cally rainless,  the  effects  produced  by  alternate  heat- 
ing and  cooling  are  very  marked.  The  rocks  are 
cracked  and  shattered  to  a  depth  of  several  inches ; 
the  surfaces  peel  off,  and  are  rapidly  disintegrated 
and  pulverised.  Wind  then  catches  up  the  loose  ma- 
terial and  sweeps  it  away,  leaving  fresh  surfaces  ex- 
posed to  the  destructive  action  of  insolation.  More 
than  this,  the  grit,  sand,  and  dust  carried  off  by  the 
wind  are  used  as  a  sand-blast  to  attack  and  erode  the 
rocks  against  which  they  strike.  In  this  manner  cliffs 
and  projecting  rocks  are  undermined,  and  masses  give 
way  and  fall  to  the  ground,  where,  subject  to  the  same 
grinding  action,  especially  towards  the  base,  they 
eventually  assume  the  appearance  of  irregular  blocks 
supported  upon  pedestals.  Mushroom-shaped  rocks 
and  hills  of  this  kind  are  common  in  all  desiccated 
rocky  regions. 

The  transporting  action  of  the  wind,  or  "  deflation," 
as  it  is  termed,  goes  on  without  ceasing  day  and  night 
and  during  all  seasons  ;  and  the  result  is  seen  in  the 
deeply  eroded  rocks,  enormous  masses  of  which,  it 
can  be  shown,  have  been  thus  gradually  removed. 
The  evidence  of  denudation  is  conspicuous,  but  its 
products  have  for  the  most  part  been  carried  away. 
In  some  places,  as  Professor  Walther  remarks  of  the 
Libyan  Desert,  are  great  walls  of  granite  rising  to 
heights  of  6000  feet,  but  showing  no  slopes  of  dd- 
bris  below,  as  would  infallibly  be  present  under  tem- 
perate conditions  of  climate.  In  other  places,  again, 


AGENTS  OF  DENUDATION  25 

are  deeply  excavated  wadies  containing  no  beds  of 
gravel,  grit,  and  sand,  such  as  would  not  fail  to  show 
themselves  had  the  depressions  in  question  been 
formed  by  water-action  alone.  Everywhere,  deep, 
cave-like  hollows  have  been  worn  out  in  the  rocks, 
and  yet  these  hold  no  sediment  or  detritus,  but  are 
swept  bare.  The  wind  tends,  in  short,  to  transport 
all  loose  material  from  the  scene  of  its  origin  to  the 
borders  of  the  desert. 

In  latitudes  like  our  own,  insolation  doubtless  shares 
in  the  disintegration  of  rocks,  but  the  most  conspicu- 
ous agent  employed  in  that  work  is  rain.  Rain 'is  not 
chemically  pure,  but  always  contains  some  proportion 
of  oxygen  and  carbonic  acid  absorbed  from  the  atmo- 
sphere ;  and  after  it  reaches  the  ground  organic  acids 
are  derived  by  it  from  the  decaying  vegetable  and 
animal  matter  with  which  soils  are  more  or  less  im- 
pregnated. Armed  with  such  chemical  agents,  it 
attacks  the  various  minerals  of  which  rocks  are  com- 
posed, and  thus,  sooner  or  later,  these  minerals  break 
up.  The  felspars  and  their  ferro-magnesian  associ- 
ates, for  example,  are  decomposed — the  carbonic  acid 
of  the  rain-water  uniting  with  the  alkalies  and  alkaline 
earths  of  those  minerals  to  form  carbonates,  which 
are  cafried  away  in  solution.  The  silica  set  free  by 
this  operation  is  also  to  some  extent  removed,  while 
the  insoluble  silicate  of  alumina,  or  clay,  remains  be- 
hind. Such  insoluble  materials  are  frequently  stained 
yellow-brown  or  red,  owing  to  the  pressure  of  iron- 
oxides.  In  this  way  felspathic  rocks  gradually  crum- 


26  EARTH  SCULPTURE 

ble  down.  Thus,  granite,  gneiss,  basalt,  and  other 
rocks  largely  composed  of  felspar,  usually  show  a 
weathered  crust,  which,  according  to  the  nature  of 
the  rock  and  the  length  of  time  its  surface  has  been 
exposed,  may  vary  from  less  than  an  inch  up  to  many 
feet,  or  even  yards,  in  thickness.  Some  granites,  for 
example,  are  reduced  to  a  kind  of  gritty  clay  which 
may  be  dug  with  a  spade. 

Argillaceous  and  silicious  rocks  are  not  so  readily 
affected  by  the  chemical  action  of  rain.  Not  infre- 
quently, however,  when  the  grains  of  a  sandstone  are 
cemented  together  by  some  soluble  substance,  such  as 
carbonate  of  lime,  the  rock  will  yield  more  or  less  read- 
ily to  the  solvent  action  of  the  water.  All  calcareous 
rocks,  in  short,  tend  to  fall  an  easy  prey.  If  they 
contain  few  or  no  impurities,  they  "  weather  "  with 
little  or  no  crust  ;  the  rock  is  simply  dissolved.  Lime- 
stones, however,  are  seldom  quite  so  pure  as  this,  but 
are  usually  impregnated  in  a  greater  or  less  degree 
with  quartz,  clay,  or  other  substance,  which  after 
the  carbonate  of  lime  has  been  removed  remains 
behind  to  form  a  crust.  The  red  and  brownish 
earths  and  clays  that  so  frequently  overlie  calcar- 
eous rocks,  such  as  chalk  and  limestone,  are  simply 
the  insoluble  residue  of  masses  of  rock,  the  soluble 
portions  of  which  have  been  dissolved  and  carried 
away  by  surface-water. 

In  all  regions  where  rain  falls,  the  result  of  this 
chemical  action  is  conspicuous ;  soluble  rocks  are 
everywhere  dissolving,  while  partially  soluble  rocks 


AGENTS  OF  DENUDATION  27 

are  becoming  rotten  and  disintegrated.  In  limestone 
areas  it  can  be  shown  that  sometimes  hundreds  of 
feet  of  rock  have  thus  been  gradually  and  silently  re- 
moved from  the  surface  of  the  land.  And  the  great 
depth  now  and  again  attained  by  rotted  rock  testifies 
likewise  to  the  destructive  action  of  rain-water  perco- 
lating from  the  surface.  This  is  particularly  notice- 
able in  warm-temperate,  sub-tropical,  and  tropical 
latitudes,  where  felspathic  rocks  are  decomposed  not 
infrequently  to  depths  of  a  hundred  feet  and  more. 
In  temperate  and  northern  regions,  the  amount  of 
rotted  rock  is  rarely  so  great.  The  thicker  rock- 
crusts  of  southern  latitudes  are  supposed  to  be  due 
to  the  larger  supplies  of  organic  acids  derived  from 
the  more  abundant  vegetation.  To  some  extent  this 
is  probably  true.  But  there  is  another  reason  for  the 
relatively  meagre  development  of  rotted  rock  in  tem- 
perate and  northern  regions  generally.  Those  re- 
gions, as  we  shall  learn  later  on,  have  recently  been 
subjected  to  glacial  conditions.  Broad  areas  of  tem- 
perate Europe  and  North  America  have  been  scraped 
bare  by  ice-sheets,  resembling  those  of  Greenland  and 
the  Antarctic  Circle.  In  more  southern  latitudes, 
the  rotted  rocks  have  escaped  such  abrasion  and 
denudation,  and  hence  it  is  not  strange  that  we  should 
find  them  attaining  so  great  a  thickness.  The  decom- 
posed rock-material  encountered  in  the  northern  parts 
of  Europe  and  America  has  been  formed  for  the  most 
part  only  since  the  disappearance  of  glacial  condi- 
tions, while  in  southern  regions  rock-decay  has  gone 


28  EARTH  SCULPTURE 

on  without  interruption  ever  since  those  lands  came 
into  existence. 

The  disintegrating  action  of  rain  in  temperate 
and  high  latitudes  is  greatly  aided  by  frost,  and  the 
same  is  the  case  in  the  elevated  tracts  of  more  south- 
ern latitudes.  Rain  renders  the  superficial  portions 
of  rock  more  porous,  and  thus  enables  frost  to  act 
more  effectually  ;  while  frost,  by  widening  pores  and 
fissures,  affords  readier  ingress  to  meteoric  water. 
Water  freezing  in  soils  and  subsoils  and  in  the  inter- 
stitial pores  and  minute  fissures  of  rocks  forces  the 
grains  and  particles  asunder,  and  when  thaw  en- 
sues the  loosened  material  is  ready  to  be  carried  away 
by  rain  or  melting  snow  and  subsequently,  it  may  be, 
by  wind.  The  same  process  takes  place  on  a  larger 
scale  in  the  prizing  open  of  joints  and  the  rending 
asunder  of  rocks  and  rock-masses.  Hence  in  Arctic 
regions  and  at  high  levels  in  temperate  and  southern 
latitudes  the  wholesale  shattering  of  rocks  has  pro- 
duced immense  accumulations  of  angular  debris.  To 
such  an  extent  has  this  action  taken  place,  that  in 
some  countries  the  rocks  are  more  or  less  completely 
buried  in  their  own  ruins.  By-and-by  so  great  do 
these  accumulations  become  that  frost  is  unable  to 
get  at  the  living  rock.  The  loose  fragments,  how- 
ever, under  which  it  lies  concealed,  are  themselves 
shattered,  crumbled,  and  pulverised,  until  they  are  in 
a  condition  to  be  swept  away  by  wind  or  melting 
snow.  By  this  means  the  solid  rock  again  comes 
within  reach  of  the  action  of  frost,  and  so  the  work  of 


AGENTS  OF  DENUDATION  29 

disruption  and  disintegration  continues.  The  great 
heaps  or  "  screes  "  of  rock-rubbish  which  cloak  the 
summits  and  slopes  of  our  mountains,  and  gather 
thickly  along  the  base  of  precipice  and  cliff,  have 
been  dislodged  by  frost  and  rolled  down  from  above, 
their  progress  downward  being  often  aided  by  tor- 
rential rains,  melting  snow,  and  the  alternate  freezing 
and  thawing  of  the  saturated  debris  itself. 

Some  reference  has  already  been  made  to  the  indi- 
rect action  of  plants  in  the  disintegration  of  rocks. 
The  various  humus  acids,  as  we  have  seen,  are  power- 
ful agents  of  chemical  change.  Without  their  aid 
rain-water  would  be  a  less  effective  worker.  The 
living  plants  themselves,  however,  attack  rocks,  and 
by  means  of  the  acids  in  their  roots  dissolve  out  the 
mineral  matters  required  by  the  organisms.  Further, 
their  roots  penetrate  the  natural  division-planes  of 
rocks  and  wedge  these  asunder ;  and  thus,  by  allow- 
ing freer  percolation  of  water,  they  prepare  the  way 
for  more  rapid  disintegration.  Nor  can  we  neglect 
the  action  of  tunnelling  and  burrowing  animals,  some 
of  which  aid  considerably  in  the  work  of  destruction. 
There  can  be  no  doubt,  for  example,  that  worms,  as 
Darwin  has  shown,  play  an  important  part  in  the  form- 
ation of  soil,  which  is  simply  rotted  rock  plus  organic 
matter. 

We  see,  then,  that  the  disintegration  and  decomposi- 
tion of  rocks  is  a  process  everywhere  being  carried 
on — from  the  crests  of  the  mountains  down  to  the 
sea,  and  in  every  latitude  under  the  sun.  No  exposed 


30  EARTH  SCULPTURE 

rock-surface  escapes  attack.  In  parched  deserts  as  in 
well-watered  regions,  in  the  dreary  barrens  of  the  far 
north  as  in  the  sunny  lands  of  the  south,  at  lofty  ele- 
vations as  in  low-lying  plains,  the  work  of  rock-waste 
never  ceases.  Here  it  is  insolation  that  is  the  most 
potent  agent  of  destruction ;  there  it  is  rain  aided  by 
humus  and  carbonic  acids  ;  or  rain  and  frost  combine 
their  forces  to  shatter  and  pulverise  the  rocks.  In 
latitudes  wh'ere  frost  acts  energetically,  the  most  con- 
spicuous proofs  of  rock-waste  are  the  sheets  and  heaps 
of  debris  that  are  ever  travelling  down  mountain- 
slopes,  or  gathering  at  the  base  of  cliff  and  precipice. 
In  lower  latitudes  the  most  impressive  evidence  of 
disintegration  is  the  great  thickness  attained  by  rotted 
rock  in  positions  where  it  is  not  liable  to  be  readily 
swept  away  by  running  water. 

Hitherto  we  have  been  considering  the  superficial 
parts  of  rock,  as  these  are  affected  by  weathering. 
We  are  not  to  suppose,  however,  that  the  alteration 
of  a  rock  ceases  immediately  underneath  its  crust. 
Rotted  rock  is  not  the  only  evidence  of  decay.  In 
the  case  of  felspathic  rocks,  it  is  found  that  some  of 
the  constituent  minerals,  more  especially  the  felspars, 
usually  show  traces  of  decomposition  at  depths  of 
many  feet  or  even  yards  below  the  weathered  super- 
ficial portions.  It  is  hard,  indeed,  to  get  a  specimen 
of  any  such  rock  from  the  bottom  of  our  deepest  quar- 
ries which  is  perfectly  fresh.  Water  soaks  through 
interstitial  fissures  and  pores,  and  finds  its  way  by 
joints  and  other  division-planes,  so  that  chemical  ac- 


i 


v  m  i  v  t  ros  |  I  T     I 

op 


DEN  UDA  T1ON  3  1 

tion,  with  resultant  rock-decay,  is  carried  on  at  the 
greatest  depths  to  which  water  can  penetrate.  This 
underground  water  eventually  comes  to  the  surface 
again  through  similar  joints,  etc.,  opening  upwards, 
and  thus  forms  natural  springs.  All  these  springs 
contain  mineral  matter,  derived  from  the  chemical 
decomposition  and  solution  of  rock-constituents. 
Many,  indeed,  are  so  highly  impregnated,  that  as  soon 
as  they  are  exposed  to  evaporation  they  begin  to  de- 
posit some  of  their  mineral  matter.  Thus  vast  quan- 
tities of  rock-material  are  brought  up  from  the  bowels 
of  the  earth.  To  such  an  extent  is  this  the  case  in 
certain  regions,  that  the  ground  is  undermined  and 
the  surface  not  infrequently  subsides.  In  countries 
where  calcareous  rocks  largely  predominate,  acidulated 
water  filtering  down  from  the  surface  through  fis- 
sures and  other  division-planes  has  often  licked  out  a 
complicated  series  of  tortuous  tunnels  and  galleries. 
So  far  has  this  process  been  carried  on  in  some  re- 
gions that  the  whole  rainfall  finds  its  way  into  subter- 
ranean courses,  and  the  entire  drainage  of  the  land  is 
conducted  underground.  The  dimensions  attained 
by  many  well-known  limestone  caverns,  and  the  great 
width  and  depth  of  the  channels  through  which  sub- 
terranean rivers  reach  the  sea,  help  us  to  appreciate 
the  amount  of  rock-material  which  underground  water 
is  capable  of  removing.  When  we  add  to  this  all  the 
mineral  matter  leached  out  at  the  surface  and  carried 
away  by  streams  and  rivers,  it  is  obvious  that  in 
course  of  time  the  land  cannot  fail  to  have  been  con- 


32  EARTH  SCULPTURE 

siderably  modified  by  chemical  action  alone.  In  point 
of  fact,  it  can  be  shown  that  from  the  surface  of  cer- 
tain regions  hundreds  of  feet  of  various  calcareous 
rocks  have  thus  been  gradually  removed ;  while  in 
other  cases  the  contour  of  the  ground  has  been  nota- 
bly affected  by  the  collapse  of  underground  channels 
and  chambers.  But  if  the  results  of  the  chemical 
action  of  meteoric  water  be  most  evident  in  places 
where  calcareous  rocks  predominate,  yet  the  thick- 
ness attained  in  other  countries  by  the  crusts  of  less 
soluble  rocks  shows  plainly  enough  that  the  whole 
land-surface  of  the  globe  is  affected  by  the  same 
action. 

We  may  now  consider  the  mechanical  action  of 
terrestrial  water,  by  means  of  which  the  more  or  less 
insoluble  residue  of  disintegrated  rock  is  removed. 
Weathered  rock  is  generally  very  porous,  and  is  thus 
readily  pulverised  by  frost.  Some  crusts  crumble 
away  as  they  are  formed,  while  others  adhere  more 
persistently.  On  slopes  and  in  mountain-regions 
generally,  decomposed  and  disintegrated  materials 
are  seldom  allowed  to  remain  long  in  situ — rain  and 
melting  snow  soon  sweep  away  the  finer  portions. 
Great  thicknesses  of  rotted  rock  are,  therefore,  some- 
what exceptional  in  such  places.  Where,  on  the  other 
hand,  the  land-surface  is  plain-like,  or  gently  undu- 
lating, and  the  drainage  sluggish,  weathered  materials 
are  not  so  readily  removed.  Nevertheless,  under  the 
influence  of  rain  alone,  or  of  rain  and  melting  snow, 
the  products  of  rock-waste  are  everywhere  travelling, 


AGENTS  OF  DENUDATION  33 

slowly  or  more  rapidly,  according  to  circumstances, 
from  higher  to  lower  levels.  In  temperate  latitudes, 
where  the  rainfall  is  distributed  over  the  year,  this 
transference  of  material  is  not  so  conspicuous  as  in 
countries  where  the  rainfall  is  crowded  into  a  short 
season.  Even  in  our  own  country,  however,  one  may 
observe  how  in  gently  undulating  tracts  rain  washes 
the  finer  particles  down  the  slopes  and  spreads  them 
over  the  hollows.  After  exceptionally  heavy  or  long- 
continued  rain  this  process  becomes  intensified — fine 
mud,  silt,  sand,  and  grit  are  swept  into  the  brooks 
and  streams,  and  the  swollen  rivers  run  discoloured  to 
the  sea.  Similar  floods  often  result  from  the  melting 
of  snow  in  spring.  During  such  floods  our  rivers  are 
generally  more  turbid  than  when  they  are  swollen 
merely  by  heavy  or  continuous  rain.  When  thaw  en- 
sues weathered  rock-surfaces  crumble  down,  while 
superficial  accumulations  of  disintegrated  materials 
become  more  or  less  saturated  by  melting  snow.  To 
such  a  degree  is  this  soaking  sometimes  carried,  that 
the  whole  surface  of  sloping  fields  may  be  set  in  mo- 
tion. The  soils  creep,  slide,  and  occasionally  flow. 
Not  infrequently  also  the  subsoils  and  disintegrated 
rock-surfaces  on  steep  inclinations  collapse  and  slide 
into  the  valleys.  Everyone,  in  short,  is  familiar  with 
the  fact  that  flooded  rivers  are  invariably  muddy,  and 
that  the  mud  or  silt  which  discolours  them  has  been 
abstracted  from  the  land. 

In  temperate  lands  of  small  extent  like  England  the 
rivers  are  under  ordinary  conditions  somewhat  clear. 


34  EARTH  SCULPTURE 

But  in  continental  tracts  the  larger  rivers  are  always 
more  or  less  turbid.  This  is  due  to  many  causes. 
Some  rivers,  for  example,  head  in  glaciers,  and  are 
thus  clouded  at  their  very  origin.  Others,  again, 
cross  several  degrees  of  latitude,  and  traverse  differ- 
ent climatic  regions.  Hence  it  will  rarely  happen 
that  snow  is  not  melting  or  rain  falling  in  some  part 
of  a  great  drainage-area.  Many  rivers,  again,  after 
escaping  from  the  mountains,  flow  through  countries 
the  superficial  formations  of  which  are  readily  under- 
mined and  washed  away,  and  thus  the  main  stream 
and  its  affluents  become  clouded  with  sediment.  It  is 
in  tropical  and  subtropical  latitudes,  of  course,  that 
the  most  destructive  effects  of  rain  are  witnessed. 
During  the  wet  season  the  rivers  of  such  regions  dis- 
charge enormous  volumes  of  mud-laden  water. 

We  may  conclude,  then,  that  under  the  influence  of 
atmospheric  agents  rocks  are  everywhere  decomposed 
and  disintegrated  ;  and,  further,  that  there  is  a  uni- 
versal transference  from  higher  to  lower  levels  of  the 
materials  thus  set  free.  Now  and  again,  it  is  true, 
there  may  be  long  pauses  in  the  journey — the  materi- 
als may  linger  in  hollows  and  depressions.  Eventu- 
ally, however,  they  are  again  put  in  motion,  and  by 
direct  or  circuitous  route,  as  the  case  may  be,  find 
their  way  into  the  rivers,  and  finally  come  to  rest  in 
the  ocean.  The  river-systems  of  the  world,  then,  are 
the  lines  along  which  the  waste  products  of  the  land 
are  carried  seawards.  But  rivers  are  much  more  than 
mere  transporters  of  sediment.  Just  as  in  desert 


AGENTS  OF  DENUDATION  35 

lands  wind  employs  disintegrated  rock-material  as  a 
sand-blast,  so  rivers  use  their  stones,  grit,  and  sand 
as  tools  with  which  to  rasp,  file,  and  undermine  the 
rocks  over  which  they  flow.  In  this  way  their  chan- 
nels are  gradually  deepened  and  widened.  Some  of 
the  transported  material  is  held  in  solution,  part  is 
carried  in  mechanical  suspension,  and  another  portion 
is  pushed  and  rolled  forward  on  the  bed.  It  is  the 
solid  ingredients,  of  course,  that  act  as  eroding  agents. 
While  much  of  the  finer  sediment  finds  its  way  into 
the  drainage-system  by  the  agency  of  rain  and  melt- 
ing snow,  the  coarser  materials  are  derived  chiefly 
from  the  destruction  of  the  rocks  that  underlie  or 
overhang  the  course  of  a  river  and  its  feeders.  In 
temperate  and  northern  latitudes  natural  springs  and 
frost  are  responsible  for  much  of  the  rock  debris  which 
cumbers  the  beds  of  streams,  but  much  also  is  dis- 
lodged by  the  undermining  action  of  the  water  itself. 
Rock-fragments  when  first  introduced  are  more  or  less 
angular,  but  as  they  travel  down  stream  they  often 
break  up  into  smaller  pieces  along  natural  cracks  or 
joints,  and  the  sharp  corners  and  edges  of  these  get 
worn  away  by  mutual  attrition,  and  by  rasping  on  the 
rocky  bed.  In  this  manner  the  several  portions  gradu- 
ally become  smoothed  and  rounded — the  process  of 
abrasion  resulting  necessarily  in  the  production  of 
grit,  sand,  silt,  etc.  Thus  in  a  typical  river-course, 
consisting  of  mountain-track,  valley-track,  and  plain- 
track,  we  note  a  progressive  change  in  the  character 
of  the  sediments  as  the  river  is  followed  from  its 


36  EARTH  SCULPTURE 

source  to  the  sea.  In  the  mountain-track,  where  the 
course  is  steep  and  usually  in  a  rocky  channel,  angular 
and  subangular  fragments  abound,  and  the  -detri- 
tus generally  is  coarse.  In  the  valley-track,  the  inclin- 
ation of  which  is  gentle,  well-rounded  gravel,  with 
grit  and  sand,  predominate,  the  latter  becoming  more 
plentiful  as  the  plain-track  is  approached.  In  the 
plain-track  the  prevailing  sediments  are  fine  sand 
and  silt. 

The  amount  of  material  removed  by  a  river  de- 
pends on  the  volume  of  the  water,  the  velocity  of  the 
current,  and  the  geological  character  of  the  drainage- 
area.  Thus,  the  larger  the  river,  other  things  being 
equal,  the  greater  the  burden  of  sediment.  Again, 
a  rapid  current  transports  material  more  effectively 
than  a  gentler  stream,  while  rivers  that  flow  through 
lands  whose  rocks  are  readily  eroded  carry  more 
sediment  than  rivers  of  equal  volume  and  velocity 
traversing  regions  of  more  resistant  rocks.  Should  a 
lake  interrupt  the  current  of  a  river,  all  the  gravel, 
sand,  and  mud  may  be  intercepted,  and  the  stream 
will  then  issue  clear  and  pellucid  at  the  lower  end  of 
the  lake,  as  the  Rhone  does  at  Geneva.  The  lake,  in 
short,  acts  as  a  settling  reservoir.  By  and  by,  how- 
ever, the  lacustrine  hollow  becomes  silted  up  and  con- 
verted into  an  alluvial  flat,  through  which  the  silt-laden 
water  winds  its  way  towards  the  ocean.  Reaching 
that  bourn,  the  current  of  the  river  is  arrested,  and 
its  sediment  thrown  down.  Should  no  strong  tidal 
current  sweep  the  coast,  removing  sediment  as  it  ar- 


AGENTS  OF  DENUDATION  37 

rives,  the  sea  becomes  silted  up  in  the  same  way  as 
the  lake,  and  in  time  a  delta  is  formed.  The  growth 
of  the  latter  necessarily  depends  partly  on  the  activ- 
ity of  the  river  and  partly  upon  the  depth  of  the 
estuary  and  the  action  of  waves  and  tidal  currents. 
But  if  nothing  interrupted  the  growth  of  a  delta- 
were  all  the  materials  brought  down  by  a  river  to  ac- 
cumulate at  its  mouth — it  is  obvious  that  the  rate  of 
increase  of  a  delta  would  enable  us  to  form  an  esti- 
mate of  the  rate  at  which  the  drainage-area  of  the 
river  was  being  eroded.  It  is  certain,  however,  that 
such  conditions  never  obtain.  Even  in  the  quietest 
estuaries  much  of  the  sediment  is  carried  away  by  the 
sea.  The  rate  of  delta-growth  must  be  exceeded  by 
that  of  fluviatile  transport. 

Geologists,  however,  have  adopted  another  method 
of  estimating  the  loss  sustained  by  the  land.  They 
can  measure  the  amount  of  material  held  in  solution, 
and  of  solid  matter  carried  in  suspension  and  rolled 
forward  on  the  bed  of  a  river.  As  might  have  been 
expected,  the  amount  varies  with  the  season  of  the 
year  in  each  individual  river,  while  different  rivers 
yield  very  different  results.  But  even  in  the  case  of 
the  least  active  streams  the  transported  material  is 
much  more  considerable  than  might  have  been  sup- 
posed. Hence  one  need  not  wonder  that  in  spite  of 
obstacles  the  deltas  of  many  rivers  advance  seawards 
more  or  less  rapidly.  The  delta  of  the  Rhone,  for  ex- 
ample, pushes  forward  at  the  rate  of  about  50  feet 
annually,  while  that  of  the  Po  increases  by  more  than 


38  EARTH  SCULPTURE 

70  yards,  and  that  of  the   Mississippi  by  80  to    100 
yards  in  the  same  time. 

It  is  sufficiently  obvious  that  the  material  carried 
seawards  by  rivers  must  afford  some  indication  of  the 
rate  at  which  the  surface  of  the  land  is  being  lowered 
by  subaerial  action.  Having  ascertained  the  annual 
amount  discharged  by  any  individual  river,  we  learn, 
at  the  same  time,  to  what  extent  the  drainage-area  of 
that  river  is  being  denuded.  In  the  case  of  the  Mis- 
sissippi, for  example,  it  has  been  calculated  that  the 
amount  of  sediment  removed  is  equal  to  a  lowering 
of  the  whole  drainage-area  by  -g-^-^th  of  afoot.  In 
other  words,  could  we  gather  up  all  the  material 
discharged  in  one  year,  and  distribute  it  equally  over 
the  wide  regions  drained  by  that  river  and  its  tributa- 
ries, we  should  raise  the  land-surface  by  g^^th  of  a 
foot.  That  does  not  seem  to  be  much,  but  at  this  rate 
of  erosion  one  foot  of  rock  will  be  removed  from  the 
Mississippi  basin  in  6000  years  ;  and  the  Mississippi  is 
not  so  active  a  worker  as  many  other  rivers.  An  aver- 
age of  many  estimates  of  the  similar  work  performed 
by  rivers  in  all  quarters  of  the  globe  shows  that  the 
rate  at  which  drainage-areas  generally  are  being  low- 
ered is  one  yard  in  8000  to  1 1,000  years.  It  must  not 
be  supposed  that  this  erosion  is  equal  throughout  any 
drainage-area.  As  a  rule,  denudation  will  take  place 
most  rapidly  over  the  more  steeply  inclined  portions 
of  the  ground.  On  mountain  declivities  and  hill 
slopes  rock-disintegration  and  the  removal  of  waste 
products  will  proceed  more  actively  than  upon  low 


AGENTS  OF   DENUDATION  39 

grounds  and  plains.  The  work  of  erosion  will  be 
carried  on  most  effectively  in  the  torrential  tracts  of 
streams  and  rivers.  Indeed,  we  may  say  that  it  is  in 
valleys  generally  that  we  may  expect  to  find  the  most 
cogent  evidence  of  erosion  now  in  action. 

A  little  consideration  will  show  that  the  estimates 
just  referred  to  do  not  tell  us  all  the  truth  concerning 
denudation.  They  show  us  only  the  amount  of  waste 
material  which  is  swept  into  the  sea.  They  afford  no 
indication  of  the  actual  amount  of  rock-disintegration 
and  erosion.  Rock-rubbish  gathers  far  more  rapidly 
in  mountain-regions  than  it  can  be  removed  by  run- 
ning water.  Indeed,  over  a  whole  land-surface  rocks 
are  disintegrated  and  debris  accumulates  from  year  to 
year.  Nor  is  the  amount  of  material  brought  down 
by  a  river  to  its  mouth  an  index  even  to  the  activity 
of  the  river  itself  as  a  denuding  and  transporting 
agent.  Enormous  volumes  of  detritus  are  deposited 
in  valleys  or  come  to  rest  in  lakes  and  inland  seas. 

Hitherto  we  have  been  treating  of  the  work  done 
by  the  atmosphere  and  running  water.  Some  refer- 
ence has  also  been  made  to  frost  as  a  potent  disin- 
tegrator of  rocks.  But  we  have  still  to  consider  the 
action  of  glaciers  in  modifying  the  surfaces  over  which 
they  flow.  It  can  be  shown  that  valleys  have  been 
widened  and  deepened,  and  broad  areas  more,  or  less 
remodelled,  by  flowing  ice,  so  that  glaciers  must  not 
be  ignored  in  any  general  account  of  denuding  agents. 
It  will  be  more  convenient,  however,  to  leave  them 
for  the  present ;  for  however  interesting  and  import- 


40  EARTH  SCULPTURE 

ant  their  action  may  be,  it  is  yet  of  minor  consequence 
so  far  as  the  origin  of  surface-features  as  a  whole  is 
concerned.  For  similar  reasons  we  may  delay  the  con- 
sideration of  marine  erosion.  The  action  of  the  sea 
upon  the  land  is  necessarily  confined  to  a  narrow  belt, 
whereas  that  of  the  subaerial  agents  affects  the  whole 
surface  of  the  land. 

We  may  take  it  that  the  denudation  of  the  surface, 
rendered  everywhere  so  conspicuous  by  the  discon- 
tinuity of  strata,  has  been  effected  mainly  by  the  at- 
mosphere and  running  water.  Other  agents  have, 
no  doubt,  played  a  part,  but  those  just  referred  to 
must  be  credited  with  the  chief  share  in  the  work  of 
erosion.  Such  is  the  general  conclusion  to  which  we 
are  led  by  the  study  of  causes  now  in  action.  And 
observation  and  reflection  combine  to  assure  us  that 
subaerial  erosion  has  been  equally  effective  during 
the  formation  of  all  the  derivative  rocks  which  enter 
so  largely  into  the  framework  of  the  earth's  crust. 
For  these  rocks  are  for  the  most  part  of  sedimentary 
origin — they  tell  us  of  ancient  lakes,  estuaries,  and 
seas.  All  their  materials  have  been  derived  from  the 
degradation  of  old  land-surfaces,  partly  no  doubt  by 
the  sea,  but  in  chief  measure  by  subaerial  agents. 
And  the  great  thickness  and  extent  attained  by  many 
of  the  geological  systems  enable  us  to  form  some  idea 
of  what  is  meant  by  denudation.  What,  for  instance, 
shall  we  say  of  a  system  composed  essentially  of  sed- 
imentary strata  reaching  a  thickness  of  several  thou- 
sand feet,  and  occupying  an  area  of  many  thousand 


AGENTS   OF  DENUDATION  41 

square  miles  ?  Obviously,  the  materials  of  such  a 
system  have  been  derived  from  the  waste  of  ancient 
lands.  Mountain-masses  must  have  been  disinte- 
grated, and  removed  in  the  form  of  sediment,  and 
gradually  piled  up,  layer  upon  layer,  on  the  floor  of 
the  sea.  Every  bed  of  sedimentary  rock,  in  short,  is 
evidence  of  denudation. 

Further,  it  has  been  ascertained  that  in  the  build- 
ing up  of  the  various  great  geological  systems  the 
same  materials  have  been  used  over  and  over  again. 
Sediments  accumulated  upon  the  sea-bottom  have 
subsequently,  owing  to  crustal  movements,  entered 


FIG.  6.     SECTION  ACROSS  UNCONFORMABLE  STRATA. 

a  «,  beds  of  sandstone,  shale,  etc.  ;  b  <5,  conglomerates  and  sandstone  resting  discordantly 
or  unconformably  upon  a  a  ;  u  u,  line  of  unconformity. 

into  the  formation  of  a  new  land-surface,  and  there- 
after, attacked  by  the  epigene  agents  of  change,  have 
again  been  swept  down  to  sea  as  gravel,  sand,  and 
mud.  The  history  of  such  changes  is  easily  read  in 
the  rock-structure  known  as  unconformity.  In  the 
accompanying  section  (Fig.  6),  for  example,  two  sets 
of  strata  are  shown — the  upper  (b)  resting  discord- 
antly or  unconformably  upon  the  lower  (a).  The 
lower  series  of  sandstones  and  shales  is  charged  with 
the  remains  of  marine  and  brackish-water  organisms 


42  EARTH  SCULPTURE 

and  of  land-plants.  The  overlying  strata  (K)  are  like- 
wise of  aqueous  origin,  and  consist  chiefly  of  con- 
glomerates and  sandstones  below,  and  of  somewhat 
finer-grained  sedimentary  beds  above.  Like  the  older 
series  (#),  they  likewise  contain  marine  and  brackish- 
water  fossils.  The  beds  (a)  introduce  us  to  an  estu- 
ary, or  shallow  bay  of  the  sea,  into  which  sediment  is 
carried  from  some  adjacent  land.  The  whole  series 
has  evidently  been  deposited  in  water  of  no  great 
depth,  as  is  shown  by  the  character  of  the  rocks  and 
their  fossil  contents.  And  as  the  strata  attain  a 
thickness  of  more  than  2000  feet,  we  must  infer  that 
during  their  accumulation  the  sea-floor  was  slowly 
subsiding,  the  rate  of  sedimentation  probably  keep- 
ing pace  with  the  subsidence.  In  other  words,  the 
bed  of  the  sea  appears  to  have  been  silted  up  as  fast 
as  it  sank,  so  that  relatively  shallow-water  conditions 
persisted  during  the  deposition  of  the  land-derived 
sediments.  Then  a  time  came  when  the  sea-floor 
ceased  to  sink  and  another  movement  of  the  crust 
took  place,  which  resulted  in  the  folding  of  the  sedi- 
mentary strata  and  the  conversion  of  the  sea-bottom 
into  dry  land.  The  folded  rocks  were  now  subjected 
during  some  prolonged  period  to  the  action  of  the 
various  subaerial  agents  of  erosion,  whereby  the  whole 
land-surface  was  eventually  denuded  and  planed  down. 
When  the  work  of  erosion  had  been  so  far  completed, 
the  entire  region  again  subsided,  and  formed  the  bed 
of  a  shallow  sea.  Under  these  conditions  the  drowned 
land-surface  became  overspread  in  time  with  new  ac- 


AGENTS   OF  DENUDATION  43 

cumulations  of  sediment,  derived  from  the  degradation 
of  adjacent  areas  that  still  continued  above  sea-level. 
The  strata  (b)  are  in  point  of  fact  largely  composed  of 
materials  derived  from  the  breaking  up  and  disinte- 
gration of  the  underlying  series  (a),  just  as  the  latter 
have  themselves  been  derived  from  the  demolition  of 
pre-existing  rock-masses.  After  the  formation  of  the 
upper  series  (ti)  the  region  was  re-elevated,  and  once 
more  formed  a  land-surface,  which  has  doubtless  en- 
dured for  a  long  period,  seeing  that  much  erosion 
has  taken  place,  the  horizontal  beds  having  been 
greatly  denuded,  trenched,  and  furrowed,  so  that  at 
the  bottom  of  deep  valleys  the  underlying  older  series 
has  been  laid  bare  and  eaten  into  by  running  water. 

Such  is  the  kind  of  tale  which  one  may  read  almost 
everywhere.  The  very  existence  of  sedimentary  strata 
implies  denudation  of  land-areas — denudation  and 
sedimentation  go  hand  in  hand.  When  we  bear  in 
mind  that  the  average  thickness  of  the  sedimentary 
rocks  which  overspread  so  large  an  area  of  the  dry 
lands  of  the  globe  cannot  be  less  than  8000  or  10,000 
feet,  we  cannot  fail  to  be  impressed  with  the  magni- 
tude of  denudation.  And  this  impression  will  be 
deepened  when  we  reflect  that  the  bulk  of  the  mate- 
rials entering  into  the  composition  of  the  derivative 
rocks  has  been  used  over  and  over  again.  The  mere 
thickness  of  existing  sedimentary  strata,  therefore,  is 
very  far  indeed  from  being  an  index  to  the  amount 
of  erosion  which  has  been  effected  since  the  deposition 
of  the  oldest  aqueous  strata. 


•  CHAPTER  III 

LAND-FORMS  IN  REGIONS  OF  HORIZONTAL 
STRATA 

VARIOUS  FACTORS  DETERMINING  EARTH  SCULPTURE — INFLUENCE 
OF  GEOLOGICAL  STRUCTURE  AND  THE  CHARACTER  OF  ROCKS  IN 
DETERMINING  THE  CONFIGURATION  ASSUMED  BY  HORIZONTAL 
STRATA — PLAINS  AND  PLATEAUX  OF  ACCUMULATION. 

TTITHERTO  we  have  been  considering  erosion 
1  1  from  one  point  of  view  only.  We  glanced  first 
at  the  general  evidence  of  denudation  as  furnished  by 
the  abrupt  truncation  and  discontinuity  of  strata,  and 
by  the  appearance  at  the  surface  of  rocks  which  could 
never  have  originated  in  that  position.  Then  we  dis- 
cussed the  action  of  existing  agents  of  change,  and 
saw  reason  to  conclude  that  the  denudation  every- 
where conspicuous  must  be  the  result  of  that  action. 
Some  reference  has  also  been  made  to  the  fact  that 
rocks  are  of  various  composition  and  consistency,  and 
therefore  tend  to  yield  and  crumble  away  unequally. 
It  follows  from  this  that  denudation  will  be  retarded 
or  hastened  according  as  the  rocks  succumb  slowly  or 
more  rapidly  to  the  action  of  eroding  agents.  Given 
an  elevated  plane-surface  of  some  extent,  composed 

44 


LAND-FORMS  IN  HORIZONTAL   STRATA       45 

of  rocks  of  different  degrees  of  durability,  and  it  is 
obvious  that  such  a  surface  must  in  time  become 
irregularly  worn  away.  The  readily  eroded  rocks 
will  disappear  most  rapidly,  and  thus  by  and  by  the 
plane-surface  will  be  more  or  less  profoundly  modified 
and  come  to  assume  a  diversified  configuration.  The 
relatively  hard  and  resisting  rocks  will  determine  the 
position  of  the  high  grounds,  while  the  low  grounds 
will  practically  coincide  with  the  areas  occupied  by 
the  more  yielding  rock-masses. 

This  we  shall  find  holds  true  to  a  large  extent  of 
all  land-surfaces.  Nevertheless,  existing  configura- 
tions have  not  been  determined  solely  by  the  min- 
eralogical  composition  of  the  rocks.  There  is  yet 
another  factor  to  be  taken  into  consideration.  The 
form  assumed  by  a  land-surface  under  denudation  de- 
pends not  only  on  the  composition  of  rocks,  but  very 
largely  on  the  mode  of  their  arrangement.  Certain 
rock-structures,  as  we  shall  learn,  favour  denudation, 
while  others  are  more  resisting.  So  dominant,  indeed, 
has  been  the  influence  of  geological  structure  in  de- 
termining the  results  worked  out  by  erosion,  that 
without  a  knowledge  of  the  structure  of  a  country  we 
can  form  no  reliable  opinion  as  to  the  origin  of  its 
surface-features. 

But  even  this  is  not  all.  We  have  likewise  to  con- 
sider the  geological  history  of  the  land  with  a  view  to 
ascertain  what  appearance  it  presented  when  rains  and 
rivers  were  just  beginning  the  work  of  erosion.  For 
it  is  obvious  that  the  direction  of  the  drainage  must 


46  EARTH  SCULPTURE 

have  been  determined  in  the  first  place  by  the  original 
inclination  of  the  surface. 

Once  more,  we  know  that  existing  land-surfaces 
have  often  been  disturbed  by  subterranean  action, 
and  that  such  action  has  not  infrequently  led  to  con- 
siderable modification  of  drainage-systems.  It  is 
remarkable,  however,  how  persistent  are  great  rivers 
in  maintaining  their  direction.  When  it  has  been 
once  fairly  established,  a  large  river  may  outlive  many 
revolutions  of  the  surface.  River-valleys  are  not 
seldom  older  than  the  mountain-ridges  which  they 
sometimes  traverse  ;  or,  to  put  it  in  another  way,  new 
mountains  may  come  into  existence  without  deflecting 
the  rivers  across  whose  valleys  they  may  seem  at  one 
time  to  have  extended — for  the  rivers  have  simply 
sawed  their  way  through  the  ridges  as  these  were 
being  gradually  developed. 

The  history  of  the  denudation  of  a  land-surface  is 
in  truth  often  highly  complicated  and  hard  to  read. 
Many  factors  have  aided  in  determining  the  final  re- 
sults of  erosion,  and  it  is  not  always  possible  to  assign 
to  each  its  proper  share  in  the  work.  But  we  may 
truly  say  that  the  sculpture  of  the  land — the  form  it 
has  assumed  under  denudation — has  been  determined 
mainly  by  these  three  factors  :  (a)  the  original  slope 
of  the  surface  ;  (K)  the  geological  structure  of  the 
ground  ;  and  (f)  the  character  of  the  rocks. 

Both  hypogene  and  epigene  agents,  therefore,  have 
been  concerned  in  the  evolution  of  land-forms.  In 
regions  much  disturbed  by  subterranean  action  within 


LAND-FORMS  IN  HORIZONTAL   STRATA       47 

relatively  recent  geological  times,  many  of  the  most 
striking  surface-features  are  obviously  due  to  deforma- 
tion and  dislocation  of  the  crust.  All  such  features, 
however,  sooner  or  later  become  modified  by  epigene 
action,  and  thus  it  has  come  to  pass  that  in  countries 
which  have  existed  as  dry  land  for  vast  periods  of  time, 
undisturbed  in  the  later  stages  of  their  history  by 
crustal  movement,  the  surface-features  are  such  as 
only  epigene  action  can  account  for.  Original 
irregularities  of  the  ground,  the  result  of  hypogene 
action,  have  been  obliterated  and  replaced  by  an 
outline  wholly  due  to  denudation. 

The  existence  of  fractured  and  folded  strata  enables 
us  vividly  to  realise  the  fact  that  hypogene  action  has 
played  a  prominent  part  in  the  evolution  of  land-forms. 
Not  only  are  many  inequalities  of  the  surface  the  di- 
rect result  of  that  action,  but  even  after  such  irregu- 
larities have  been  removed,  the  various  positions 
assumed  by  the  flexed  and  fractured  rocks  have 
largely  determined  the  configuration  subsequently 
worked  out  by  the  epigene  agents  of  change.  Thus 
both  directly  and  indirectly  crustal  movements  have 
had  a  large  share  in  the  production  of  surface-features. 
It  is  not  necessary  for  our  purpose  to  inquire  into  the 
causes  of  such  movements.  In  the  opinion  of  most 
geologists  they  are  due  to  the  secular  cooling  of  the 
earth.  As  the  nucleus  cools  it  contracts,  and  the 
already  cooled  crust  sinks  down  upon  it.  This  move- 
ment necessarily  results  in  the  fracturing  and  wrinkling 
of  the  crust,  which  as  it  sinks  is  compelled  to  occupy 


48  EARTH  SCULPTURE 

a  smaller  superficial  area.  The  deformation  brought 
about  in  this  way  varies  in  extent.  In  some  places 
the  general  subsidence  of  the  crust  has  not  been 
marked  by  much  disturbance  of  the  rocks ;  the  orig- 
inal horizontality  of  the  strata  has  been  largely  pre- 
served. In  other  regions  the  reverse  is  the  case,  the 
strata  having  been  everywhere  folded  and  fractured ; 
and  between  these  two  extremes  are  many  gradations. 
The  various  structures  assumed  by  disturbed  rock- 
masses  show  that  crustal  movements  are  of  two  kinds, 
horizontal  and  vertical.  Folding  and  its  accompany- 
ing phenomena  are  obviously  the  result  of  tangential 
pressure.  Sometimes  the  strata  are  so  folded  as  to 
present  the  appearance  of  a  series  of  broad,  gentle 
undulations.  At  other  times  the  folds  are  pressed 
closely  together  and  bent  over  to  one  side  in  the 
direction  of  crustal  movement.  In  certain  regions  so 
great  has  been  the  horizontal  thrust,  that  masses  of 
rock,  thousands  of  feet  in  thickness,  have  sheared 
under  the  pressure  and  travelled  forwards  for  miles, 
older  rocks  being  pushed  forward  bodily  over  younger 
masses.  But  besides  such  horizontal  movements  there 
are  vertical  movements  of  the  crust,  typically  repre- 
sented by  the  dislocations  known  as  normal  faults. 
Normal  faults  are  more  or  less  vertical  displacements, 
often  of  small  amount,  but  not  infrequently  very 
great.  Many  are  vast  rents  traversing  the  crust  in 
some  determinate  direction,  the  rocks  on  one  side  of 
the  fault  having  subsided  for  hundreds  or  even  for 
thousands  of  feet.  We  may  reserve  for  the  present. 


LAND-FORMS  IN  HORIZONTAL   STRATA      49 

however,  any  further  discussion  of  the  rock-structures 
that  result  from  hypogene  action.  All  that  we  need 
at  present  bear  in  mind  is  the  general  fact  that  the 
crust  of  the  earth  is  subject  to  deformation. 

We  now  proceed  to  inquire  more  particularly  into 
the  influence  of  geological  structure  and  the  character 
of  rocks  upon  the  development  of  land-forms.  We 
shall  therefore  consider  first  the  form  assumed  by 
lands  built  up  of  approximately  horizontal  strata. 
This  is  the  simplest  kind  of  geological  structure  :  the 
tale  it  tells  is  not  hard  to  read.  We  can  follow  it 
from  first  to  last  in  all  its  details.  But  if  we  succeed 
in  grasping  what  is  meant  by  the  denudation  of  hori- 
zontal strata,  we  shall  have  little  difficulty  in  explaining 
the  origin  of  surface-features  in  regions  the  geological 
structure  of  which  is  much  more  complicated. 

As  common  examples  of  horizontally  bedded  strata 
we  may  take  the  alluvial  deposits  that  mark  the  sites 
of  vanished  lakes ;  the  terraces  of  gravel,  sand,  and 
silt  that  occur  in  river-valleys ;  deltas,  and  raised 
beaches.  Fluvio-marine  deposits  and  raised  beaches 
of  recent  age  generally  form  low  plains  rising  but  a 
few  feet  or  yards  above  sea-level.  Their  inclination 
is  seawards,  usually  at  so  low  an  angle  that  they  often 
appear  to  the  eye  level,  or  approximately  so.  This 
gently  sloping  surface  is  an  original  configuration,  for 
it  corresponds  with  the  structure  of  the  various  under- 
lying deposits,  the  general  inclination  or  dip  of  which 
is  in  the  same  direction  as  the  surface.  When  that 
surface  is  approximately  level  denudation  necessarily 


50  EARTH  SCULPTURE 

proceeds  very  slowly,  although  in  time  the  action  of 
rain  alone  will  suffice  to  lower  the  general  level.  But 
however  much  raised  beaches  and  deltas  of  recent  age 
may  have  been  modified  superficially  by  subaerial 
denudation,  we  must  admit  that  their  most  character- 
istic features  are  original,  and  due  to  the  mode  of 
their  formation. 

The  same  holds  true  to  a  large  extent  of  recent 
lacustrine  and  fluviatile  deposits.  The  wide  flats  that 
tell  us  where  lakes  formerly  existed,  and  the  broad 
alluvial  tracts  through  which  streams  and  rivers 
meander,  are,  like  deltas  and  raised  beaches,  plains  of 
accumulation.  It  goes  without  saying,  however,  that 
many  of  these  plains  are  more  or  less  eroded,  and 
have  acquired  an  undulating,  furrowed,  and  irregular 
surface.  Some  alluvial  tracts,  indeed,  have  been  so 
cut  up  by  rain  and  running  water  that,  in  the  rough, 
rolling  ground  over  which  he  toils,  the  traveller  may 
find  it  hard  to  recognise  the  characteristic  features  of 
a  plain. 

In  a  broad  river-basin  alluvial  terraces  and  plains 
usually  occur  at  various  heights,  marking  successive 
levels  at  which  the  river  and  its  tributaries  have  flowed 
while  deepening  their  courses.  The  lowest  terraces 
and  flood-plains  are,  of  course,  the  youngest,  and  show, 
therefore,  least  trace  of  subaerial  erosion.  As  we  re- 
cede from  these  modern  alluvia  and  rise  to  higher 
levels,  the  terraces  and  plains  become  more  and  more 
denuded.  The  highest-lying  river-accumulations,  in- 
deed, may  be  so  much  eaten  into  and  washed  down 


LAND-FORMS  IN  HORIZONTAL   STRATA       51 

that  only  scattered  patches  may  remain,  and  few  or  no 
traces  of  the  original  flat  surface  can  then  be  recog- 
nised. Thus  fluviatile  terraces  and  recent  alluvia  all 
tend  to  become  modified  superficially,  while  at  the 
same  time  they  are  undermined  and  cut  into  by 
streams  and  rivers. 

The  plains  of  accumulation  at  present  referred  to 
belong  to  a  recent  geological  age,  and  consist  for  the 
most  part  of  incoherent  deposits,  such  as  gravel,  sand, 
clay,  silt,  loam,  and  so  forth.  And  it  is  worthy  of 
note  that  the  nature  of  the  deposits  has  to  some  ex- 


FIG.  7.     SECTION  ACROSS  A  SERIES  OF  ALLUVIAL  TERRACES. 

r,  solid  rocks  ;  i,  oldest  terrace  ;   2,  second  terrace  ;  3,  third  and  youngest  terrace ;  4,  river  and 
recent  alluvial  plain. 

tent  influenced  the  denudation  of  the  ground.  Thus 
terraces  and  plains  composed  mainly  of  gravel  tend 
to  retain  their  original  level  surface,  while  similar 
flats  of  clay  and  loam  of  the  same  age  as  the  gravel 
have  frequently  been  furrowed  and  channelled  to  such 
an  extent  that  the  originally  level  surface  has  largely, 
or  even  entirely,  disappeared.  The  reason  is  obvious, 
for  clay  and  loam  are  somewhat  impervious,  while 
gravel  is  highly  porous.  Consequently  rain  falling  on 
the  surface  of  the  latter  is  rapidly  absorbed,  and  little 
or  no  superficial  flow  is  possible.  But  in  the  case  of 
the  more  impervious  deposits  rain  is  absorbed  very 


52  EARTH  SCULPTURE 

sparingly,  and  naturally  tends  to  produce  inequalities 
as  it  seeks  its  way  over  the  gently  inclined  surface. 

The  origin  and  present  aspect  of  such  recent  plains 
of  accumulation  are  so  obvious  and  so  readilyaccounted 
for,  that  it  is  hardly  necessary  to  do  more  than  cite  a 
few  examples.  Amongst  the  most  notable  are  the 
great  deltas  of  such  rivers  as  the  Mississippi,  the 
Amazon,  the  Rhone,  the  Po,  the  Danube,  the  Rhine, 
the  Niger,  the  Ganges,  etc.,  and  the  broad  flats  and 
terraces  which  occur  within  the  drainage-areas  of  the 
same  rivers.  The  vast  plains  of  the  Aralo-Caspian 
area,  and  the  far-extended  tundras  of  Northern  Siberia, 
are  likewise  examples  of  plains  of  accumulation,  all 
of  which  belong  to  recent  geological  times.  How- 
ever much  some  of  these  plains  may  have  been 
furrowed  and  trenched  by  running  water,  we  yet  have 
no  difficulty  in  recognising  that  the  general  form  of 
the  surface  is  due  to  sedimentation.  The  deposits  of 
which  they  are  built  up  have  been  laid  down  in 
approximately  horizontal  or  gently  inclined  layers,  and 
the  even  or  level  surface  is  thus  simply  an  expression 
of  the  arrangement  of  the  bedding.  In  a  word,  the 
geological  structure  has  determined  the  configuration 
of  the  surface. 

But  it  is  needless  to  say  that  horizontal  strata  are 
not  confined  to  low  levels,  nor  do  they  always  consist 
of  unconsolidated  materials,  like  gravel,  sand,  and 
clay.  Horizontal  strata  of  such  rocks  as  sandstone, 
shale,  limestone,  basalt,  etc.,  enter  largely  into  the 
composition  of  certain  lofty  plateaux  and  mountain- 


LAND-FORMS  IN  HORIZONTAL   STRATA       53 

regions.  And  they  belong,  moreover,  to  very  differ- 
ent geological  periods,  some  being  of  comparatively 
recent  formation,  while  others  date  back  to  ages 
incalculably  remote. 

One  of  the  most  interesting  and  instructive  regions 
of  the  kind  is  the  remarkable  plateau  of  the  Grand 
Canon  district  of  Arizona  and  Utah.  This  plateau 
occupies  an  area  of  between  13,000  and  16,000 
square  miles,  and  is  traversed  by  the  Colorado  River 
of  the  West,  which  follows  a  tortuous  course  tow- 
ards west-south-west  through  a  succession  of  pro- 
found ravines  or  canons.  The  strata  visible  at  the 
surface  are  approximately  horizontal,  and  attain  a 
thickness  of  many  thousand  feet.  It  may  be  said, 
therefore,  that  the  prevalent  plain-like  character  of 
the  surface  is  an  expression  of  the  underground  struct- 
ure— that,  in  short,  the  Grand  Canon  district  is  a 
plateau  of  accumulation.  This,  in  a  broad  sense,  is 
doubtless  true  ;  but  when  we  come  to  examine  the 
configuration  and  structure  of  the  district  more  closely, 
we  find  reason  to  conclude  that  the  original  surface 
has  been  greatly  modified  by  denudation.  We  learn, 
moreover,  that  the  strata  are  not  quite  horizontal. 
The  inclination  is  certainly  gentle,  but  a  slope  of  only 
one  degree,  if  continued  for  a  few  miles,  will  result 
in  a  fall  of  several  hundred  feet.  If  a  surface  be  in- 
clined at  an  angle  of  one  degree,  then  for  every 
eleven  miles  of  distance  it  will  lose  1000  feet  of  ele- 
vation. Now,  in  the  Grand  Caflon  district  the  gen- 
eral inclination  of  the  strata  is  towards  north  and 


54 


EARTH  SCULPTURE 


o  £ 

1 1 

O        O 

£,    c 

1 


§  I 

s  I 

o'    2 


U.         V 

o    -3, 


-  ? 

«  S 

w  | 

c«  o 

Q  -5 

04  •" 

«  -o 


00          g, 

J  f 


north-east,  while  the  slope 
of  the  surface  is  in  the 
opposite  direction.  Thus 
it  comes  to  pass  that  strata 
which  lie  open  to  the  day 
upon  the  south-west  mar- 
gin of  the  plateau  gradu- 
ally descend  towards  north 
and  north-east,  until,  in  a 
distance  of  120  miles  or 
thereabouts,  they  lie  bur- 
ied at  a  depth  of  several 
thousand  feet.  It  is  not 
quite  true,  therefore,  that 
in  the  Grand  Canon  dis- 
trict the  form  of  the 
ground  is  an  exact  expres- 
sion of  the  underground 
structure.  On  the  con- 
trary, the  average  slope 
of  the  surface  is  against 
and  not  with  the  average 
dip  of  the  strata.  Never- 
theless ,  it  cannot  be 
doubted  that  the  general 
configuration  of  the  re- 
gion— its  plateau-charac- 
ter— has,  in  the  first  place, 
been  determined  by  the 
approximately  horizontal 


LAND-FORMS  IN  HORIZONTAL   STRATA       55 

disposition  of  the  strata,  and  that  it  may  be  rightly 
termed  a  plateau  of  accumulation.  A  glance  at  the 
geological  history  of  the  district  will  show  how  far  the 
plateau-character  is  original,  and  to  what  extent  and 
by  what  means  it  has  been  subsequently  modified. 

Reference  has  been  made  to  the  fact  that  the  rocks 
composing  the  plateau  are  chiefly  of  aqueous  origin, 
and  approximately  horizontal. 

Here  and  there  in  the  bottoms  of  deep  canons  we 
get  peeps  at  another  set  of  rocks  that  form  the 
pavement  upon  which  the  horizontal  strata  repose. 
With  the  history  of  these  older  underlying  rocks  we 
need  not  concern  ourselves  further  than  to  note 
that  they  are  of  Pre-Cambrian  and  early  Palaeozoic 
age.  It  is  with  the  superincumbent  masses  that  we 
have  to  deal.  Those  attain  a  vast  thickness,  and 
range  in  age  from  Carboniferous  down  to  Eocene 
times.  At  the  beginning  of  the  Carboniferous  Period 
the  district  formed  a  portion  of  the  sea-floor,  and 
similar  marine  conditions  obtained  during  the  deposi- 
tion of  all  the  succeeding  systems  of  strata  down  to 
the  close  of  Cretaceous  times.  Throughout  all  that 
long  succession  of  ages  the  sea  would  appear  never  to 
have  been  deep,  although  during  the  early  part  of  the 
Carboniferous  Period  it  was  probably  deeper  than  in 
subsequent  times.  When  we  consider  that  the  marine 
sediments  reach  a  united  thickness  of  over  15,000  feet 
it  may  at  first  sight  appear  impossible  that  so  thick  a 
mass  of  materials  could  accumulate  in  a  shallow  sea. 
The  explanation,  however,  is  simple  enough — sub- 


56  EARTH  SCULPTURE 

sidence  kept  pace  with  sedimentation.  Slowly  and 
gradually  the  bed  of  the  sea  went  down — slowly  and 
gradually  it  was  silted  up  by  sediments  derived  from 
the  adjacent  land. 

At  last,  towards  the  close  of  Cretaceous  times,  cer- 
tain new  crustal  movements  began — elevation  ensued, 
and  the  sea  finally  retired  from  the  district.  An  ex- 
tensive lake  now  occupied  the  site  of  the  plateau- 
country,  for  a  prolonged  period,  during  which  sediments 
were  washed  down  as  before  from  the  neighbouring 
uplands,  and  gathered  over  the  level  surface  of  the 
Cretaceous  marine  strata  until  they  had  reached  a 
thickness  of  5000  feet  or  more.  As  these  deposits 
appear  likewise  to  have  been  laid  down  chiefly  in 
shallow  water,  it  may  be  inferred  that  the  slow  subsid- 
ence of  the  area  which  accompanied  the  accumula- 
tion of  the  underlying  marine  strata  was  repeated 
during  the  lacustrine  period. 

The  whole  region,  it  will  be  understood,  had  been 
elevated  at  the  close  of  Cretaceous  times ;  but  the 
movement  was  differential,  the  greatest  rise  having 
been  experienced  by  the  uplands  surrounding  the  la- 
custrine basin.  Eventually  the  river,  escaping  over 
the  lower  lip  of  that  basin,  deepened  the  outlet  and 
succeeded  in  draining  the  lake,  which  was  then  re- 
placed by  an  alluvial  plain.  At  this  stage  the  nearly 
level  surface  of  the  drained  lake-bed  sloped  gently 
from  east-north-east  to  west-south-west,  and  thus  de- 
termined the  direction  of  the  primeval  Colorado 
River  and  its  larger  tributaries,  which  headed  then 


LAND-FORMS  IN  HORIZONTAL   STRATA       57 

as  now  in  the  high  lands  overlooking  the  basin. 
When  these  waters  first  began  to  wander  across  the 
alluvial  plain,  the  slope  of  the  surface  and  the  inclina- 
tion of  the  underlying  sedimentary  strata  doubtless 
coincided.  But  these  conditions  were  ere  long  dis- 
turbed by  successive  movements  of  elevation,  and  the 
prevalent  horizontality  of  the  strata  was  modified. 
Here  and  there  the  beds  were  bent  or  flexed,  and 
traversed  by  great  fractures  along  which  the  strata 
became  vertically  displaced  for  thousands  of  feet. 
Yet,  strange  to  say,  none  of  these  earth-movements 
succeeded  in  deflecting  the  main  drainage  of  the  dis- 
trict. The  Colorado  and  its  chief  affluents  continued 
to  flow  in  the  courses  they  had  attained  at  the  final 
disappearance  of  the  great  lake.  It  is  clear,  there- 
fore, that  the  bending  and  dislocation  of  the  strata 
must  have  proceeded  very  slowly,  for  the  rivers  were 
able  to  cut  their  way  across  both  flexures  and  faults 
as  fast  as  these  showed  at  the  surface. 

Before  the  great  lake  had  vanished  some  portions 
of  the  older  marine  strata  had  been  elevated,  and 
formed  part  of  the  land  surrounding  the  basin.  Here 
they  were  for  a  long  period  exposed  to  the  erosive 
action  of  epigene  agents,  and  must  have  suffered 
much  loss.  But  all  such  denudation  sinks  into  insig- 
nificance when  we  consider  the  magnitude  of  the 
erosion  which  has  taken  place  since  the  great  lake 
dried  up.  Fortunately,  owing  to  the  simple  geologi- 
cal structure  of  the  Grand  Canon  district,  the  amount 
of  that  erosion  can  be  readily  estimated.  According 


EARTH  SCULPTURE 

to  Captain    Dutton,    the    average 

thickness  of  strata  removed   from 

^    an  area  of  13,000  to  15,000  square 

|    miles  cannot  have  been  under  10,- 

2  ooo  feet.    This  may  seem  a  startling 
conclusion,  but  it  is  based  on  evi- 

o 

%  dence   which   cannot   be  gainsaid. 

'o  Throughout     the     major     portion 
of   the    plateau-country  horizontal 

t  Carboniferous    strata    occupy   the 

?.  surface.      As    these    are    followed 

|  northward    they  gradually    dip    in 

J;  that  direction  under  younger  strata 

|  (Permian,  Mesozoic,  and  Cainozoic 

|  rocks),  until  they  are  buried  at  last 

f  to  a  depth  of  10,000  feet  and  more. 

j>  Now  Captain   Dutton  has  shown 

1  that  this  vast  thickness    of   over- 

**f  lying    strata     formerly     extended 

;•  throughout      the      whole      Grand 

i  Canon  district.     This  is  proved  by 

I  the  fact  that  many  outliers  or  relics 

>•    of  the  rocks   in   question   still   re- 

£  •        .  ,          .  , 

z    mam,  scattered   at  intervals   over 

§    the    broad    surface    of    the    Car- 

I    boniferous      strata.       They     form 

conspicuous  table-shaped  and  pyr- 

3  amidal    hills,   rising  more    or  less 
a    abruptly  above  the  great  Carbon- 
iferous platform.     The  accompany- 
ing   diagram    shows    the    general 


LAND-FORMS  IN  HORIZONTAL   STRATA       59 

relations  of  those  isolated  "buttes"  and  "mesas,"  as 
they  are  termed,  to  the  underlying  Carboniferous 
rocks  and  the  strata  at  T,  of  which  they  are  detached 
outliers.  The  dotted  line  (a-ti)  indicates  the  level 
originally  attained  by  the  plateau.  All  the  rock  that 
formerly  existed  between  a-b  and  the  surface  of  the 
Carboniferous  strata  (C)  has  been  denuded  away. 

How  has  this  enormous  erosion  been  effected,  and 
what  are  the  more  prominent  features  of  the  denuded 
area  ?  A  low-lying  plain  of  accumulation,  such  as  a 
delta,  cannot  experience  much  erosion ;  the  surface 
is  approximately  level,  or  has  only  a  very  gentle  in- 
clination, and  any  water  flowing  over  it  must  be 
sluggish  and  ineffective.  But  conceive  such  a  plain 
upheaved  for  several  hundred  feet,  and  it  is  obvious 
that  the  fall  of  the  river  to  the  sea  will  then  be  in- 
creased and  its  erosive  action  greatly  augmented.  It 
will  therefore  proceed  to  dig  a  deeper  and  wider 
course  for  itself.  Now  let  us  suppose  that  an  ele- 
vated plain  is  traversed  not  by  one  main  river  only, 
but  by  numerous  affluents,  each  with  its  quota  of 
tributary  streams.  The  running  waters  will  continue 
to  deepen  their  channels  until  the  gradient  by  the  pro- 
cess is  gradually  reduced  to  a  minimum  and  vertical 
erosion  ceases.  The  main  river  will  be  the  first  to 
attain  this  base-level — a  level  not  much  above  that  of 
the  sea.  The  plain-track  will  gradually  extend  from 
the  sea  inland  until  the  same  low  gradient  is  attained 
throughout  the  whole  course  of  the  river.  In  time 
all  the  affluents  with  their  tributaries  will  arrive  at  the 
same  stage. 


60  EARTH  SCULPTURE 

But  rivers  do  not  only  cut  vertically  ;  they  also  un- 
dermine their  banks  and  cliffs,  and  thus  erode  hori- 
zontally ;  hence  it  follows  that  the  valleys  will  be 
widened  as  well  as  deepened.  The  widening  process 
may  be  greatly  aided  by  the  action  of  wind,  rain, 
springs,  and  frost.  Not  infrequently,  indeed,  these 
agents  play  as  important  a  part  as  the  streams  them- 
selves. Under  the  conditions  now  described  an  ele- 
vated plain  will  in  course  of  time  be  cut  up  into  more 
or  less  numerous  segments,  the  upper  surfaces  of 
which  will  represent  the  original  level  of  the  land  ; 
where  the  interval  between  two  valleys  is  wide  we 
shall  have  a  broad,  flat-topped  segment ;  where  the 
interval  is  short  the  segment  will  be  correspondingly 
restricted  in  size.  In  a  word,  the  segments  will  vary 
in  extent  according  to  the  multiplicity  and  intricacy 
of  the  valley-system. 

A  word  now  as  to  the  form  of  the  slopes  and  cliffs 
bounding  the  valleys.  We  are  dealing,  it  will  be  re- 
membered, with  an  elevated  plain  of  accumulation. 
The  horizontal  strata,  we  shall  suppose,  are  more  or 
less  indurated  beds  of  conglomerate,  sandstone,  shale, 
and  limestone.  All  rocks,  as  we  have  seen,  are 
traversed  by  natural  division-planes  or  joints,  and  these 
in  the  case  of  stratified  rocks  consist  of  two  sets 
intersecting  each  other  and  the  planes  of  bedding  at 
approximately  right  angles.  Horizontal  strata  are  in 
this  way  divided  up  into  rudely  cuboidal,  quadrangu- 
lar, or  rectangular  blocks.  Joints  are,  of  course,  lines 
of  weakness  along  which,  when  rocks  are  undermined, 


LAND-FORMS  IN  HORIZONTAL   STRATA       61 

they  tend  to  give  way.  Thus  when  horizontal  strata 
are  cut  into  by  rivers  and  undermined  they  break  off 
at  the  joints,  and  vertical  cliffs  result.  It  does  not 
often  happen,  however,  that  in  a  considerable  series 
of  strata  all  the  beds  are  of  quite  the  same  character. 
Frequently  some  are  relatively  harder  and  unyielding, 
while  others  are  softer  and  more  readily  reduced. 
Let  us  suppose  that  the  uppermost  bed  cut  into  by 
the  river  is  somewhat  hard  and  difficult  to  grind 
through.  In  time  the  water  saws  its  way  down  into 
the  succeeding  stratum,  which  we  shall  take  to  be  a 
soft  or  easily  eroded  shale.  In  the  overlying  hard 
rock  the  river  has  been  able  to  cut  merely  a  narrow 
steep-sided  trench.  The  shale,  however,  offers  much 
less  resistance  to  the  vertical  and  lateral  action  of  the 
water,  and  is  thus  rapidly  intersected  and  washed 
away  from  underneath  the  superincumbent  harder 
stratum.  The  latter,  losing  its  support,  then  yields 
along  its  joint-planes,  and  a  larger  or  smaller  slice  is 
detached  from  the  wall  of  the  cliff  and  falls  in  ruins. 
In  this  way  the  cliffs  gradually  retire  as  they  are  un- 
dermined— in  a  word,  the  ravine  is  not  only  deepened 
but  widened. 

Much  of  the  rock  debris  dislodged  from  the  cliffs 
falls  into  the  river,  and  is  gradually  broken  up  and 
carried  away ;  but  some  comes  to  rest  at  the  base, 
forming  a  talus,  and  thus  retards  the  denudation  of 
the  shale.  To  the  action  of  the  river  we  must  add 
that  of  other  epigene  agents,  such  as  wind,  rain, 
springs,  and  frost,  under  the  influence  of  which  the 


62 


EARTH  SCULPTURE 


shale  weathers  away  more  rapidly  than  the  overlying 
rock,  and  eventually  forms  a  sloping  stage  upon 
which  the  debris  derived  from  the  receding  cliffs 
continues  to  accumulate.  Meanwhile,  however,  the 
river  digs  down  through  the  shale  and  encounters,  we 
shall  suppose,  another  thick  stratum  of  hard  rock. 
Lateral  erosion  by  the  running  water  is  now  reduced 
to  a  minimum  ;  slowly  the  current  saws  its  way  down 


FIG.   10.      DIAGRAMMATIC  SECTION   SHOWING  STAGES  OF  EROSION  BY   A 
RIVER  CUTTING  THROUGH  HORIZONTAL  STRATA.    (After  Captain  Button.) 

A,  relatively  hard  rocks ;  s,  relatively  soft  strata ;  r  r,  river  at  successive  stages  as  valley 
is  deepened  and  widened. 

vertically,  just  as  it  did  in  the  uppermost  unyielding 
bed,  until  it  again  reaches  a  second  layer  of  shale. 
The  undermining  action  is  now  repeated,  and  a  sec- 
ond line  of  rock-wall  begins  to  retreat  in  the  same 
manner  as  the  first.  And  so  the  process  goes  on  with 
all  the  succeeding  strata  through  which  the  river  cuts, 
until  it  finally  attains  a  minimum  gradient  and  ceases 
to  erode.  But  note  that,  while  the  deepening  of  the 
ravine  proceeds,  the  cliffs  never  cease  to  retire.  Each 


LAND-FORMS  IN  HORIZONTAL   STRATA       63 

individual  layer  of  softer  rock  continues  to  waste 
away  more  rapidly  than  the  harder  bed  above  it. 
Thus  eventually  a  river-valley  appears  bounded,  not 
by  vertical  cliffs,  but  rather  by  a  succession  of  hori- 
zontal tiers  of  precipitous  faces,  corresponding  to 
the  outcrops  of  the  several  strata  of  harder  rock — 
separated  the  one  from  the  other  by  the  longer  or 
shorter  slopes  yielded  by  the  shales. 

Finally,  we  may  further  note  that  the  recession  of 
the  cliffs  will  be  much  influenced  by  the  rate  at  which 
their  basal  portions  are  undermined.  Each  slice  re- 
moved from  a  steep  rock-face  narrows  the  width  and 
increases  the  inclination  of  the  sloping  stage  above. 
Hence,  as  Captain  Button  has  clearly  shown  in  his 
admirable  description  of  the  Colorado  Canons,  the  de- 
scent of  debris  from  each  stage  is  facilitated,  while  the 
weathering  of  the  soft  rocks  and  the  undermining  of 
the  overlying  harder  beds  are  accelerated.  Thus, 
curiously  enough,  as  the  same  author  remarks,  the 
state  of  affairs  at  the  bottom  influences  the  rate  of 
recession  at  the  summit. 

When  a  river  has  reached  its  base-level  and  ceases 
to  erode,  the  valley-slopes  and  cliffs,  nevertheless, 
under  the  influence  of  weathering,  continue  to  retire. 
The  debris  showered  down  from  above  now  tends  to 
accumulate  below,  and  thus  affords  protection  to  the 
rocks  against  which  it  is  banked.  And  the  talus  thus 
formed  continues  to  rise  higher  and  higher.  The  ex- 
posed strata  above,  however,  having  no  such  protec- 
tion, weather  as  before,  each  rock-tier  retreating,  but 


64  EARTH  SCULPTURE 

at  a  gradually  diminishing  rate.  What  form  the 
ground  will  ultimately  assume  will  largely  depend 
upon  climatic  conditions.  If  the  climate  be  moist  and 
frost  be  active  in  winter,  the  sharp  edges  of  the  rock- 
tiers  will  be  bevelled  off,  and  the  sloping  surfaces  will 
become  heavily  laden  with  debris  and  disintegrated 
rock-material,  the  further  degradation  and  removal  of 
which  will  be  retarded  by  the  growth  of  vegetation. 
Thus,  in  time,  the  sharp  angles  will  tend  to  disappear, 
and  a  somewhat  undulating  slope  will  replace  the 
more  strongly  marked  features  which  the  same  rocks 
would  have  yielded  under  arid  conditions. 

Let  us  now  recall  what  was  said  as  to  the  cutting 
up  of  our  elevated  plain  into  a  multiplicity  of  flat- 
topped  segments,  and  we  shall  see  reason  to  conclude 
that  these  segments  must  be  bounded  by  steep  faces, 
the  aspect  of  which  will  vary  according  to  the  nature 
of  the  strata  and  the  character  of  the  climate.  If  the 
climate  be  arid,  and  the  strata  consist  of  alternate 
hard  and  soft  beds  of  variable  thickness,  the  bound- 
ing walls  of  the  segments  may  in  some  places  be  ap- 
proximately vertical,  or  they  may  show  a  succession 
of  short  cliffs  with  intermediate  sloping  stages.  If, 
on  the  other  hand,  the  climate  be  moist,  those  features 
will  be  more  or  less  softened  and  modified.  In  the 
former  case  step-like  profiles  will  abound  ;  in  the  latter 
the  ground  will  likewise  ascend  in  stages,  but  these 
will  be  less  accentuated,  and  may  even  be  in  large 
part  replaced  by  continuous  slopes.  Again,  each  flat- 
topped  segment  of  the  denuded  area,  eaten  into  on  all 


LAND-FORMS  IN  HORIZONTAL   STRATA       65 

sides,  will  continually  contract,  the  bounding  cliffs 
and  slopes  retiring  step  by  step  until  they  eventually 
meet  atop.  The  flat  summit  now  disappears,  and  is 
replaced  by  a  sharp  crest,  ridge,  peak,  or  rounded 
top,  as  the  case  may  be.  Each  diminishing  segment, 
in  short,  ultimately  acquires  a  more  or  less  strongly 
pronounced  pyramidal  form.  This,  however,  is  not 
the  final  stage.  Denudation  continues — pyramidal 
hills,  dome-shaped  heights,  and  crested  ridges  gradu- 
ally crumble  down,  until  at  last  all  abrupt  and  pro- 
minent irregularities  of  surface  disappear,  and  the  once 
elevated  plain  returns  to  its  former  state,  that  of  a 
gently  undulating  or  approximately  flat  stretch  of 
low-lying  land.  The  cycle  of  erosion  is  completed. 

Thus  in  the  erosion  of  a  plateau  of  horizontal  strata 
we  recognise  the  following  stages  : — (i)  The  excava- 
tion of  deep  trenches  by  streams  and  rivers ;  (2)  the 
gradual  sapping  and  undermining  of  cliffs,  etc.,  the 
widening  of  valleys,  and  the  consequent  cutting  up  of 
the  plateau  into  a  multitude  of  flat-topped  blocks  or 
segments ;  (3)  the  progressive  contraction  of  the  seg- 
ments, and  their  conversion  into  pyramidal  or  round- 
topped  hills  and  crested  ridges  ;  and  (4)  the  continued 
reduction  and  lowering  of  the  hills  and  final  resolution 
of  the  plateau  into  a  plain. 

This  plain,  in  the  hypothetical  case  we  have  been 
considering,  is  supposed  to  be  at  a  level  very  little 
above  that  of  the  sea.  But  the  minimum  level  to 
which  a  region  tends  to  be  reduced  need  not  be  at 
such  a  low  elevation.  The  streams  and  rivers  dis- 


66  EARTH  SCULPTURE 

charging  into  a  great  lake  or  inland  sea  cannot  erode 
their  valleys  below  the  level  of  the  quiet  water  which 
is  the  receptacle  of  their  sediment.  That  surface 
becomes  for  them  a  base-level  of  erosion,  and  all 
their  energies  are  employed  in  the  task  of  reducing 
to  that  level  the  land  over  which  they  flow.  Soon  or 
late,  however,  the  outlet  of  the  lake  will  be  deepened, 
the  surface  of  the  latter  will  fall,  and  the  base-level 
will,  of  course,  be  lowered  at  the  same  time.  But 
should  a  slow  movement  of  elevation  affect  the  lower 
end  of  the  great  lake,  and  thus,  by  counterbalancing 
the  work  of  river  erosion  at  its  outlet,  maintain  the 
surface  at  approximately  the  same  level  for  a  pro- 
longed period  of  time,  then  denudation  may  eventually 
succeed  in  reducing  to  that  base-level  all  the  lands 
that  drain  into  the  lake.  The  lake  might  be  entirely 
silted  up,  but  so  long  as  the  movement  of  elevation 
persisted,  and  the  river  (at  the  former  outlet  of  the 
lake)  continued  to  saw  its  way  down  as  rapidly  as  the 
ground  was  upheaved,  the  old  base-level  of  erosion 
would  be  maintained. 

We  may  now  return  to  the  Grand  Canon  district 
and  the  question  of  its  erosion.  During  the  progress 
of  the  great  denudation  the  interior  spaces  of  the  dis- 
trict, according  to  Captain  Button,  "  occupied  for  a 
time  the  relation  of  an  approximate  base-level  of 
erosion."  The  whole  region  has  been  greatly  ele- 
vated, but  this  upheaval  was  not  effected  all  at  one 
time.  On  the  contrary,  in  place  of  one  single  con- 
tinuous movement  a  succession  of  uplifts  has  taken 


LAND-FORMS  IN  HORIZONTAL   STRATA       67 

place,  each  separated  from  the  other  by  a  period  of 
repose.  It  was  during  one  of  these  prolonged  pauses 
that  enormous  sheets  of  strata,  averaging  some  10,000 
feet  in  thickness,  were  gradually  broken  up  and  re- 
moved from  the  surface  of  the  Carboniferous  rocks, 
while  the  latter  themselves  were  planed  down  to  a  flat 
expanse.  This  Carboniferous  platform  served  for  a 
long  time  as  a  base-level  of  erosion.  The  horizontal 
masses  under  which  it  lay  buried  were  first  deeply 
incised  by  the  Colorado  River  and  its  affluents  and 
their  countless  tributaries.  The  strata  thus  became 
broken  up  into  innumerable  separate  blocks  or  seg- 
ments, which,  little  by  little,  were  reduced  in  size  and 
most  of  them  eventually  demolished.  But  before  the 
last  remaining  "  buttes  "  and  "  mesas  "  could  be  re- 
moved a  great  change  supervened.  A  general  up- 
heaval of  the  entire  area  for  several  thousand  feet 
took  place,  and  the  base-level  to  which  the  district 
had  been  so  largely  reduced  was  destroyed.  The 
gradients  of  all  the  rivers  now  increased,  and  the 
velocity  of  the  currents  was  correspondingly  aug- 
mented, with  the  result  that  the  erosion  of  ravines 
and  caftons  recommenced. 

It  is  beyond  the  purpose  of  these  pages  to  trace 
further  the  history  of  the  Grand  Cafton  district.  But 
those  who  wish  to  have  an  adequate  conception  of 
what  is  meant  by  river  erosion  would  do  well  to  con- 
sult Captain  Button's  work.  From  it  they  will  learn 
how  the  Colorado  River  has,  within  a  very  recent 
geological  period,  dug  out  a  valley  "  more  than  200 


68  EARTH  SCULPTURE 

miles  long,  from  5  to  1 2  miles  wide,  and  from  5000  to 
6000  feet  deep."  From  our  present  point  of  view  the 
chief  lesson  which  we  derive  from  a  study  of  the 
Grand  Canon  district  is  simply  this :  that  horizontally 
arranged  strata  tend  under  the  action  of  epigene 
agents  to  form  flat-topped  mesas  and  pyramidal  hills 
and  mountains.  The  contours  of  those  prominent 
features  and  the  detailed  sculpturing  of  cliffs  and 
rock-terraces  will  depend  largely  upon  the  character 
of  the  strata  out  of  which  the  hills  and  mountains  are 
carved,  and  also  to  a  great  extent  upon  the  climate. 
In  a  dry  elevated  tract  like  that  of  the  Canon  district 
the  influence  exerted  by  the  petrological  character  of 
the  strata  in  determining  the  detailed  features  of  the 
ground  is  everywhere  conspicuous.  In  other  regions 
where  moister  climatic  conditions  prevail  this  influ- 
ence, although  never  absent,  is  yet  not  so  strongly 
marked. 

In  the  foregoing  discussion  the  configuration  as- 
sumed by  horizontal  strata  has  been  dealt  with  in 
such  detail  that  it  is  not  necessary  to  cite  more  than 
a  few  other  examples  to  show  that  wherever  the  same 
geological  structure  occurs  denudation  has  resulted 
in  the  production  of  similar  land-forms. 

The  lonely  group  of  the  Faroe  Islands,  lying  about 
half-way  between  Scotland  and  Iceland,  are  the  relics 
of  what  at  one  time  must  have  been  a  considerable 
plateau.  They  extend  over  an  area  about  seventy 
miles  in  length  from  north  to  south,  and  nearly  fifty 
miles  in  width  from  east  to  west.  The  original 


LAND-FORMS  IN  HORIZONTAL   STRATA       69 


plateau  could  not  have  been  less  than 
3500  square  miles  in  extent.  But  as 
the  islands  have  everywhere  experienced 
excessive  marine  erosion,  it  is  certain 
that  the  plateau  out  of  which  they  have 
been  carved  formerly  occupied  a  much 
wider  area.  The  geological  structure 
of  the  islands  is  very  simple.  They  are 
built  up  of  a  great  succession  of  basalts 
with  thin  intervening  layers  of  tuff 
(volcanic  dust,  etc.)  arranged  in  ap- 
proximately horizontal  strata.  The 
islands  are  for  the  most  part  high  and 
steep,  many  of  them  being  mere  mount- 
ain-ridges that  sink  abruptly  on  one  or 
both  sides  into  the  sea.  The  larger 
ones  show  more  diversity  of  surface, 
but  possess  very  little  level  land.  All 
have  a  mountainous  character,  and, 
owing  to  the  similarity  of  the  rocks  and 
their  arrangement,  exhibit  little  variety 
of  feature.  They  form  as  a  rule  strag- 
gling, irregular,  flat-topped  masses,  and 
sharper  ridges,  that  are  notched  or 
broken  here  and  there  into  a  series  of 
isolated  peaks  and  truncated  pyramids. 
Sometimes  the  mountains  rise  in  gentle 
acclivities,  but  more  generally  they  show 
steep  and  abrupt  slopes,  which  in  several 
instances  have  inclinations  of  25°  to 


70  EARTH  SCULPTURE 

27°  or  even  30°.  In  many  places  they  are  yet  steeper, 
their  upper  portions  especially  becoming  quite  pre- 
cipitous. They  everywhere  exhibit  a  well-marked 
terraced  character  ;  precipices  and  long  walls  of  bare 
rock  rise  one  above  another,  like  the  tiers  of  some 
cyclopean  masonry,  and  are  separated  usually  by 
short  intervening  slopes,  sparsely  clothed  with  grass 
and  moss,  or  sprinkled  with  tumbled  rock-rubbish. 
The  coasts  are  usually  precipitous,  many  of  the  islands 
having  only  a  few  places  where  a  landing  can  be 
effected.  Not  a  few  are  girt  by  cliffs,  ranging  in 
height  from  200  or  300  feet  up  to  1000  feet,  and  even 
in  some  places  exceeding  2000  feet.  The  best-defined 
valleys  are  broad  in  proportion  to  their  length.  Fol- 
lowed up  from  the  head  of  a  sea-loch,  they  rise  some- 
times with  a  gentle  slope  until  in  the  distance  of  two 
or  three  miles  they  terminate  in  a  broad  amphitheatre- 
like  cirque.  In  many  cases,  however,  they  ascend  to 
the  water-parting  in  successive  broad  steps  or  terraces. 
Each  terrace  is  cirque-shaped,  and  framed  in  by  a 
wall  of  rock,  the  upper  surface  of  which  stretches 
back  to  form  the  next  cirque-like  terrace,  and  so  on 
in  succession  until  the  series  abruptly  terminates  at 
the  base,  it  may  be,  of  some  precipitous  mountain. 
Occasionally  the  neck  between  two  valleys  running 
in  opposite  directions  is  so  low  and  flat  that  it  is  with 
difficulty  that  the  actual  water-parting  can  be  fixed. 
In  such  cases  we  have  a  well-defined  hollow,  bounded 
by  precipitous,  terraced  hill-slopes,  crossing  an  island 
from  shore  to  shore.  Were  the  land  to  be  slightly 


LAND-FORMS  IN  HORIZONTAL   STRATA       71 

depressed  such  hollows  would  form  sounds  separating 
adjacent  islands,  while  the  valleys  that  head  in  cirques 
would  form  sea-lochs.  There  can  be  no  doubt,  in- 
deed, that  the  existing  fiords  of  the  Faroes  simply 
occupy  the  lower  reaches  of  land-valleys,  and  that  the 
sounds  separating  the  various  islands  from  each  other 
in  like  manner  indicate  the  sites  of  long  hollows  of 
the  character  just  described.  In  a  word,  the  islands 
are  the  relics  of  a  plateau  of  comparatively  recent 
geological  age,  for  the  rocks  date  no  further  back 
than  Oligocene  times.  All  the  land-features  are  the 
result  of  subaerial  erosion  guided  and  determined  by 
the  petrological  character  and  horizontal  arrangement 
of  the  strata.  The  precipitous  cliffs  of  the  coast-line 
owe  their  origin,  of,  course,  to  the  undermining  action 
of  the  sea,  the  rocks  ever  and  anon  giving  way  along 
the  well-marked  vertical  joint-planes. 

In  Great  Britain  horizontal  strata  occupy  no  broad 
areas.  But  wherever  they  put  in  an  appearance 
they  yield  the  same  surface-features.  Thus  in  the 
north-west  Highlands  we  have  the  striking  pyrami- 
dal mountains  of  Canisp,  Suilven,  and  Coulmore, 
carved  out  of  horizontal  red  sandstones  of  Pre-Cam- 
brian  age.  In  Caithness,  again,  we  have  the  peaked 
and  truncated  pyramids  of  Morven,  Maiden  Pap,  and 
Smean,  hewn  out  of  approximately  horizontal  Old  Red 
Sandstone  strata.  Ingleborough  is  another  good 
example  of  a  pyramidal  mountain  having  a  similar 
geological  structure.  Many  illustrations  are  likewise 
furnished  by  the  horizontal  strata  of  other  lands. 


72  EARTH  SCULPTURE 

Thus  pyramidal  and  more  or  less  abrupt  hills,  the 
precipitous  sides  of  which  are  defined  by  vertical  joints, 
are  common  in  the  horizontally  bedded  "  Quadersand- 
stein  "  of  Saxon  Switzerland.  So  again  in  the  region 
of  the  Dolomites,  whenever  the  strata  are  horizontal 
the  mountains  carved  out  of  them  tend  to  assume 
pyramidal  forms.  In  a  word,  we  may  say  that  all  the 
world  over  the  same  geological  structure  gives  rise  to 
the  same  land-forms. 

River-courses  hewn  in  horizontal  strata  will  vary 
somewhat  in  form  according  to  the  nature  of  the 
rocks  and  the  character  of  the  climate.  In  regions 
built  up  of  relatively  unyielding  rocks,  or  of  alterna- 
tions of  these  and  less  resisting  beds,  the  valleys  tend 
to  be  trench-like,  and  the  mountain-slopes  are  more 
or  less  abrupt.  But  under  the  influence  of  rain, 
springs,  and  frost  these  harsh  features  are  toned 
down,  river-cliffs  are  benched  back,  and  abrupt  ac- 
clivities are  replaced  by  gentler  slopes.  Should  the 
strata  consist  of  soft  materials  throughout,  there  will 
be  a  general  absence  of  harsh  features  ;  round-topped 
hills  and  moderate  valley-slopes  will  characterise  the 
land.  Nevertheless,  whether  the  strata  be  "  hard  "  or 
"soft,"  thick-bedded  or  thin-bedded,  or  show  alterna- 
tions of  many  different  kinds,  and  whether  the  climate 
be  arid  or  humid,  equable  or  the  reverse — tropical, 
temperate,  or  arctic — the  same  general  type  of  surface- 
features  can  always  be  recognised. 


CHAPTER  IV 

LAND-FORMS  IN  REGIONS  OF  GENTL  Y  INCLINED 

STRATA 

ESCARPMENTS  AND  DIP-SLOPES — DIP-VALLEYS  AND  STRIKE-VAL- 
LEYS— FORMS  ASSUMED  BY  A  PLATEAU  OF  EROSION — VARIOUS 
DIRECTIONS  OF  ESCARPMENTS — SYNCLINAL  HILLS  AND  ANTI- 
CLINAL HOLLOWS — ANTICLINAL  HILLS. 

THE  most  characteristic  land-forms  met  with  in 
regions  where  the  strata  are  inclined  in  some 
general  direction  are  escarpments  and  dip-slopes,  the 
former  coinciding  with  the  outcrops,  and  the  latter 
with  the  inclination  or  dip  of  the  strata.  In  such 
regions  some  streams  and  rivers  not  infrequently 
flow  in  the  direction  of  dip,  and  thus  cut  across  the 
escarpments,  while  others  may  traverse  the  land  along 
the  base  of  the  escarpments. 

The  origin  of  these  phenomena  is  not  hard  to  trace. 
Let  us  suppose  that  some  wide  tract  of  horizontal 
strata  has  been  elevated  along  an  axis  so  as  to  form 
a  considerable  island.  If  the  movement  of  elevation 
were  slowly  effected  the  sea  would  doubtless  modify 
the  land-surface  as  it  arose,  but  for  simplicity's  sake 
we  shall  ignore  such  action,  and  suppose  that  the 
new-born  land  exists  as  an  elongated  island,  the  sur- 

73 


74  EARTH  SCULPTURE 

face  sloping  away  at  a  low  angle  on  either  side  of  a 
somewhat  flattened  axis.  (Fig.  12.)  At  first,  then, 
the  surface  coincides  with  the  underground  structure 
— a  dome-shaped  land  formed  of  dome-shaped  strata. 
(Fig.  13.)  It  is  obvious  that  the  drainage  will  be  in 


FIG.  12.     MAP  OF  AN  ISLAND  COMPOSED  OF  DOME-SHAPED  STRATA. 

The  strata  are  inclined  in  the  direction  of  the  arrows. 

the  direction  of  the  dip  of  the  strata — all  the  main 
rivers  will  take  the  quickest  route  to  the  sea.  But  as 
we  cannot  suppose  that  the  surface  of  the  new-made 
land  would  be  without  some  irregularities,  the  streams 
and  rivers  would  not  actually  follow  straight  courses. 


FIG.  13.     SECTION  THROUGH  THE  ISLAND  SHOWN  IN  FIG.  12. 

Slopes  of  surface  coincide  with  arrangement  of  strata. 

On  the  contrary,  it  could  not  but  happen  that  one 
stream  would  eventually  join  another,  and  in  this  way 
many  might  become  tributaries  of  one  or  more  large 
rivers.  Thus  we  should  have  certain  courses  cut  in 
the  general  direction  of  the  dip,  while  others  joining 
these  would  in  some  places  go  with  the  inclination  of 


LAND-FORMS  IN  GENTL  Y  INCLINED  STRA  TA     75 

the  strata,  and  in  other  places  would  traverse  that  at 
various  angles.  The  strata  consist,  we  shall  suppose, 
of  "  hard  "  and  "  soft  "  rocks — limestones,  sandstones, 
shales,  etc.,  and  they  are  well  jointed  at  right  angles 
to  the  planes  of  bedding.  Thus,  while  the  strata  dip 
seaward,  one  set  of  joints  is  inclined  at  a  high  angle 
in  the  opposite  direction — the  other  set  cutting  the 
strata  in  the  direction  of  the  dip.  Now  so  long  as 
the  streams  follow  the  dip  it  is  obvious  that  they  will 
tend  to  form  trench-like  valleys — the  rocks  will  be 
undermined  and  give  way  along  vertical  joint-planes. 


FIG.  14.     SECTION  OF  RIVER- VALLEY. 

The  valley  coincides  in  direction  with  the  "  strike  "  of  the  strata,  i.  <».,  it  trends  at  right  angles 
to  the  dip  or  inclination  ;  </,  cliff  determined  by  joint ;  s  j,  springs ;  r,  river. 

We  need  not  for  the  present  consider  the  modifica- 
tions arising  from  the  varying  character  of  the  rocks. 
It  is  enough  to  remember  that  since  they  yield  along 
the  joint-planes,  they  tend  to  produce  vertical  or 
steeply  inclined  walls  in  the  same  manner  as  if  they 
were  horizontally  bedded.  But  when  the  course  of  a 
stream  is  more  or  less  at  right  angles  to  the  dip  of 
the  strata,  the  valley  it  forms  will  not  have  the  same 
trench-like  aspect.  On  one  side  of  such  a  valley  the 
strata  dip  away  from  the  stream,  and  when  under- 
mined they  yield  along  the  joints  which  incline  inland. 


76  EARTH  SCULPTURE 

A  cliff  thus  determined  is  not  so  liable  to  be  broken 
down  by  the  action  of  springs  and  frost.  Under- 
ground water  tends  to  move  away  down  the  dip- 
planes,  so  that  no  springs  come  out  on  the  face  of 
the  cliff  d  (Fig.  14),  which  is  only  renewed  from  time 
to  time  by  the  undermining  action  of  the  river  and 
the  consequent  collapse  of  the  rock  along  a  steeply 
inclined  joint.  On  the  opposite  side  of  the  valley 
the  conditions  are  different.  There  the  dip  is  towards 
the  river — a  weak  structure,  for  the  strata  are  easily 
undermined  and  sapped  by  springs,  coming  out  along 
the  planes  of  bedding  (s,  s).  Hence  they  readily  give 
way,  their  debris  sliding  and  rolling  towards  the  river. 
Thus  valleys  that  coincide  in  direction  with  the  out- 
crop of  the  strata  will  usually  show  a  somewhat  pre- 
cipitous cliff  on  one  side  and  a  more  or  less  gentle 
slope  on  the  other. 

We  shall  not  follow  the  subsequent  history  of  the 
erosion  of  our  island  in  any  detail.'  It  is  obvious, 
however,  that  it  must  pass  through  the  same  stages 
of  erosion  as  any  similar  area  of  horizontally  bedded 
rocks.  The  rivers  and  their  multitudinous  feeders 
will  deepen  and  widen  their  valleys  until  the  ground 
is  cut  up  into  a  more  or  less  numerous  series  of  seg- 
ments or  blocks.  But  these  will  differ  in  form  from 
those  which  are  carved  out  of  horizontal  strata.  In- 
stead of  flat-topped  mesas  and  buttes  and  pyra- 
midal-shaped hills,  we  shall  have  a  series  of  heights 
presenting  escarpments  towards  the  watershed  and 
long  slopes  in  the  opposite  direction.  (Fig.  15.) 


LAND-FORMS  IN  GENTL  Y  INCLINED  STRA  TA     7  7 

Eventually  these  will  largely  disappear,  and  the  whole 
region  will  be  resolved  into  a  gently  undulating  plain 
of  erosion. 

Now  let  us  suppose  that  this  plain  is  upheaved  and 
converted  into  a  plateau,  the  surface  of  which  has  a 
very  gentle  inclination  in  the  same  general  direction 
as  the  dip.  (See  Fig.  16,  p.  78.)  The  section  at  the 
side  of  the  map  shows  the  geological  structure.  Here 
obviously  the  surface-slope  is  not  so  great  as  the 


FIG.  15.    ENLARGED  SECTION  OF  A  PORTION  OF  THE  ISLAND  SHOWN 
IN  FIG.  12. 

Upper  dotted  line  shows  original  surface ;  e  <?,  outcrops  of  "  hard  "  beds  forming  escarpments. 

inclination  of  the  underlying  strata  ;    the  plateau  is 
therefore  a  plateau  of  erosion. 

The  map  represents  the  course  of  a  main  stream 
with  its  tributaries.  The  trend  of  the  drainage  will 
naturally  be  in  the  same  direction  as  the  dip,  and  the 
rivers  must  therefore  traverse  the  outcrops  of  the 
strata.  Were  the  surface  of  the  plateau  quite  even 
the  waters  would,  of  course,  descend  by  a  direct  route 
to  the  sea.  For  various  reasons,  however,  it  is  very 
unlikely  that  such  should  be  the  case.  The  strata 
had  no  doubt  been  planed  down  to  a  base-level,  but 
some  inequalities  would  still  exist — the  outcrops  of 
the  most  durable  rocks  would  here  and  there  project, 


78  EARTH  SCULPTURE 

however  slightly,  above  the  general  surface.  We 
may  suppose,  for  example,  that  the  outcrops  of  the 
limestones,  (e  e)  would  form  low  ridges,  rising,  it 
might  be,  only  a  few  feet  or  yards.  Such  slight 
inequalities  would  suffice,  however,  to  divert  the 
waters  to  right  or  left.  The  rivers  and  streams  being 


FIG.  16.     DIAGRAM  MAP  OF  PLATEAU  OF  EROSION. 

1 1,  low  ridges  formed  by  outcrops  of  limestone,  which  are  seen  in  section  at  the  side. 

turned  in  this  manner  out  of  their  direct  course  would 
be  compelled  to  flow  along  the  outcrops  until  depres- 
sions in  the  ridges  allowed  them  to  resume  their 
original  direction. 

After  such  a  drainage-system  had  been  well  estab- 
lished, and  the  whole  surface  of  the  land  had  been 
subjected  to  the  action  of  the  various  epigene  agents 


LAND-FORMS  IN  GENTL  Y  INCLINED  STRA  TA     79 

of  change  for  some  protracted  period  of  time,  the 
inequalities  of  surface  would  become  greatly  accent- 
uated. The  regions  occupied  by  "  softer "  rocks 
would  be  generally  lowered,  so  that  the  outcrops  of 
the  harder  beds  would  stand  up  more  and  more  promi- 
nently. These,  however,  would  not  remain  unchanged. 
On  the  contrary,  each  bed  of  hard  rock,  constantly 
undermined  by  the  wearing  away  of  the  softer  under- 
lying strata,  would  continue  to  recede  at  its  outcrop. 
This  retreat  would  be  most  marked  in  places  where 
the  rivers  flowed  along  the  base  of  the  escarpments. 
But  even  where  rivers  were  absent  the  escarpments 


FIG.  17.     SECTION  ACROSS  REDUCED  PLATEAU  OF  EROSION. 

The  upper  dotted  line  represents  original  surface  of  plateau  as  shown  in  Fig.  16. 

would  still  mark  the  outcrops  of  the  harder  beds. 
These,  no  doubt,  might  not  be  so  prominent  as  the 
others,  and  would  not  retreat  so  rapidly,  but  they 
would  nevertheless  come  to  form  striking  features  in 
the  landscape.  In  a  word,  the  region  would  eventu- 
ally be  traversed  from  left  to  right  by  pronounced 
lines  of  escarpment  rising  to  many  hundreds  of  feet 
above  the  low  grounds  at  their  base,  and  falling  away 
in  a  long  gentle  slope  in  the  direction  of  the  dip. 
When  these  land-forms  were  fully  developed  a  section 
across  the  reduced  plateau  would  show  the  structure 
seen  in  Fig.  17. 


8o 


EARTH  SCULPTURE 


In  the  case  we  have  been  considering  the  surface 
of  the  plateau  of  erosion  is  inclined  in  the  same  direc- 
tion as  the  dip  of  the  strata.  Consequently  all  the 
escarpments  face  the  water-parting  of  the  region,  and 
all  the  dip-slopes  sink  towards  the  sea.  But  the  sur- 
face of  such  a  plateau  may  be  inclined  against  the 
direction  of  the  dip  ;  the  outcrops,  instead  of  facing 
the  water-parting,  may  look  seawards.  Nevertheless, 
should  hard  beds  be  intercalated  amongst  more  yield- 
ing strata,  escarpments  are  certain  to  make  their 
appearance  under  the  influence  of  denudation,  and 


FIG.  1 8.    LONGITUDINAL  SECTION  OF  RIVER-COURSE. 

River  flowing  from  a  to  b  ;  A,  outcrop  of  hard  stratum  ;  s  j*,  shales  ;  w1,  position  of  water- 
fall when  river-bed  has  been  eroded  to  the  level  /*  ;  «/a,  position  of  waterfall  when 
river-bed  has  been  eroded  to  the  level  /a. 

may  become  quite  as  prominent  as  in  the  case  we 
have  just  been  considering.  Nor  will  the  character 
of  river-valleys  excavated  in  the  direction  of  the 
"  strike  "  of  the  strata  differ  ;  cliffs  will  tend  as  before 
to  be  developed  on  one  side,  and  gentle  slopes  on  the 
other.  But  in  the  river-courses  that  traverse  the 
strike  more  or  less  at  right  angles  we  shall  meet  with 
certain  marked  contrasts.  In  regions  where  the 
rivers  flow  in  the  same  direction  as  the  dip  of  gently 
inclined  strata,  waterfalls  are  not  readily  formed. 
When  the  outcrop  of  a  relatively  hard  bed  is  en- 
countered the  overlying  softer  rocks  may  be  rapidly 


LAND-FORMS  IN  GENTL  Y  INCLINED  STRA  TA     8 1 

washed  away,  and  the  surface  of  the  underlying  hard 
bed  be  exposed.  At  most,  however,  this  simply  gives 
rise  to  a  rapid,  which  can  only  approach  the  character 
of  a  waterfall  when  the  strata  are  inclined  at  a  high 
angle.  But  when  the  strata  dip  up-stream  the  condi- 
tions are  reversed.  The  outcrop  of  every  hard  ledge 
then  gives  rise  to  a  cascade,  and  should  the  hard  rocks 
attain  a  considerable  thickness  a  notable  waterfall  may 
be  produced.  In  the  diagram  annexed  (Fig.  18)  the 
upper  line  shows  the  course  of  a  river  (&—$)  flowing 
across  a  series  of  strata  inclined  at  a  low  angle  up- 
stream. At  h  we  see  the  outcrop  of  a  bed  of  hard 
sandstone  or  other  relatively  durable  stratum,  under- 
laid and  overlaid  by  soft  shales.  It  is  obvious  that 
the  river  cannot  lower  the  surface  of  the  overlying 
soft  shales  (sx)  much  below  the  outcrop  of  the  hard 
stratum.  So  long  as  that  endures  the  beds  at  sx  are 
safe.  It  is  otherwise,  however,  with  the  underlying 
shales  (s).  These  are  more  or  less  rapidly  eroded, 
and  in  the  process  of  their  removal  the  superjacent 
hard  stratum  is  undermined,  and  from  time  to  time 
gives  way  along  its  joint-planes.  In  this  manner  the 
waterfall  (ze/1)  gradually  retreats  further  up  the  valley 
(w*),  and  a  gorge  comes  into  existence. 

Thus  in  the  river-courses  of  a  plateau  of  erosion, 
composed  of  gently  inclined  strata  with  an  up-stream 
dip,  waterfalls  tend  to  be  developed  at  the  outcrops 
of  intercalated  hard  beds.  But,  as  erosion  proceeds, 
these  waterfalls  retreat  up  the  valley,  and  so  are 
gradually  replaced  by  gorges. 

6 


82  EARTH  SCULPTURE 

Now  it  may  be  said  generally  that  in  all  regions 
composed  of  gently  inclined  strata,  amongst  which 
relatively  hard  beds  are  intercalated,  escarpments  and 
dip-slopes  are  developed  by  denudation.  When  the 
dip  of  the  strata  is  persistent  over  a  wide  extent  of 
country,  we  shall  have  more  or  less  prominent  escarp- 
ments traversing  such  a  region  continuously  for 
miles.  The  escarpments  will  obviously  vary  in  char- 
acter with  the  angle  of  dip  and  the  nature  and  thick- 
ness of  the  rocks.  If  the  hard  bed  or  beds  be  of  no 
great  thickness  and  the  dip  high,  the  resulting  escarp- 
ment and  slope  will  constitute  a  somewhat  narrow 
ridge ;  but  if  the  thickness  of  the  hard  beds  be  very 
considerable  and  the  dip  gentle,  the  escarpment  may 
assume  the  form  of  a  belt  of  plateau  or  a  range  of  high 
ground,  having  a  more  or  less  diversified  surface. 
England  supplies  some  excellent  examples  of  the 
kind.  The  general  inclination  of  the  strata  between 
the  borders  of  Wales  and  the  North  Sea  is  easterly, 
at  a  low  angle  ;  consequently,  as  we  walk  in  that 
direction  we  cross  the  outcrops  of  several  great  geo- 
logical systems.  These  are  built  up  of  sedimentary 
rocks,  some  of  which  are  relatively  soft  and  yielding, 
such  as  clay  and  shale,  while  others  are  harder  and 
generally  more  porous,  such  as  limestone,  chalk,  etc. 
Hence  in  time  the  latter  have  come  to  form  a  series 
of  more  or  less  prominent  escarpments  or  belts  of 
high  ground,  separated  by  broad  tracts  of  gently 
undulating  low  ground.  Starting  from  the  foot  of 
the  Malvern  Hills,  and  proceeding  in  an  easterly 


LAND-FORMS  IN  GENTL  Y  INCLINED  STRA  TA     83 

direction,  we  first  traverse  low-lying  plains  of  sand- 
stone and  argillaceous  beds,  until  on  the  other  side  of 
the  Severn  we  reach  the  Cotswolds,  a  belt  of  high 
ground  over  1000  feet  in  height,  and  reaching  in 
places  a  width  of  30  miles.  The  rocks  of  which  these 
hills  are  composed  consist  principally  of  limestones, 
which,  as  they  dip  gently  eastwards,  are  succeeded  by 
a  series  of  argillaceous  beds,  forming  again  a  region 
of  undulating  plains.  Traversing  these  plains  in 
the  direction  of  dip,  we  eventually  encounter  another 
broad  belt  of  high  ground — the  escarpment  of  the 
Chalk.  This  escarpment  in  its  turn  is  succeeded  by 
a  low-lying  region  composed  chiefly  of  relatively  soft 
argillaceous  beds  and  other  non-indurated  strata. 

A  glance  at  any  geological  map  of  the  country  will 
show  that  all  the  prominent  hills  and  high  grounds 
of  central  and  south-eastern  England  are  developed 
along  the  outcrops  of  the  Jurassic  limestones  and  the 
Chalk,  and  thus  have  a  general  northerly  or  north- 
easterly trend.  We  cannot  doubt  that  the  present 
irregularities  of  the  surface  are  the  result  of  long-con- 
tinued epigene  action,  guided  by  the  character  of  the 
rocks  and  the  geological  structure  of  the  ground. 
The  yielding  strata  have  been  worn  away  more  rap- 
idly than  the  harder  rocks,  while  the  escarpments 
formed  by  the  latter  have  slowly  retreated  as  denuda- 
tion proceeded.  This  is  sufficiently  evidenced  by  the 
fact  that  detached  outliers  of  the  more  durable  beds 
are  met  with  lying  beyond  the  general  outcrop  of  the 
series.  Thus  in  Fig.  19  the  outliers  of  Chalk  (i,  2) 


84 


EARTH  SCULPTURE 


were  obviously  at  one  time  connected  with  the  main 
mass  C — the  dotted  line  representing  the  conditions  of 
surface  that  formerly  obtained.  In  a  word,  the  de- 
tached masses  have  been  left  behind  during  the  retreat 
of  the  escarpment  to  its  present  position.  The  course 
of  the  River  Thames,  whose  head-waters  rise  on  the 
east  side  of  the  Cotswold  Hills,  was  doubtless  deter- 


FIG.  19.    SECTION  OF  ESCARPMENTS  AND  OUTLIERS. 

mined  by  the  inclination  of  the  original  surface  of  the 
ground.  It  will  be  observed  that,  like  the  streams 
represented  in  Fig.  16,  this  river  flows  across  the  out- 
crops of  the  Jurassic  and  Cretaceous  strata,  cutting 
through  the  Chalk  escarpment  between  Wallingford 
and  Reading. 

Although  wide  regions  may  be  built  up  of  strata 


FIG.  20.     SECTION  ACROSS  THE  WEALDEN  AREA.     (Ramsay.) 

tf,  Upper  Cretaceous  strata  ;  £,  Lower  Greensand,  etc.;  r,  Weald  clay ;  rf,  Hastings  sands,  etc. 

dipping  continuously  in  one  direction,  yet  it  is  more 
usual  to  find  the  direction  of  dip  changing.  Such 
changes  may  occur  at  wide  intervals,  or  they  may  suc- 
ceed each  other  within  narrower  limits.  Sometimes 
we  may  have  the  beds  of  a  broad  area  arranged  in 


LAND. FORMS  IN  GENTL  Y  INCLINED  STRA  TA     85 

one  single  anticline  or  syncline  as  the  case  may  be. 
In  other  places  the  undulations  of  the  strata  may  be 
numerous.  Many  examples  of  such  structures  might 
be  cited  from  the  rocks  of  Great  Britain.  Restrict- 
ing attention  for  the  present  to  gently  inclined  and 
undulating  strata,  we  encounter  a  fine  illustration 
of  a  broad  anticline  in  the  Chalk  Downs  and  the 
Weald.  (Fig.  20.)  The  latter  might  be  described  as 
a  wide  amphitheatre,  open  to  the  sea  on  the  east,  but 
surrounded  in  all  other  directions  by  bold  bluff-like 
hills.  Here  the  configuration  has  had  precisely  the 
same  origin  as  the  escarpments  of  the  Midlands.  The 
North  and  South  Downs  coincide  with  the  outcrops 
of  the  Chalk,  while  the  enclosed  low  grounds  have 
been  excavated  out  of  underlying  argillaceous  and 
other  unconsolidated  strata.  The  Chalk,  one  cannot 
doubt,  originally  extended  over  the  whole  of  the 
Wealden  area,  as  shown  by  the  dotted  lines  in  Fig. 
20.  That  high  ground  formerly  existed  within  this 
area  is  clearly  indicated  by  the  fact  that  the  escarp- 
ment of  the  Downs  has  been  sawn  across  by  streams 
flowing  out  from  the  heart  of  the  Weald.  Obviously 
when  these  streams  first  began  to  flow,  the  water- 
parting  in  the  axis  of  the  Weald  must  have  been  at  a 
higher  level  than  the  present  summit  of  the  Downs. 
The  whole  surface  has  been  lowered  by  epigene  ac- 
tion— the  less  readily  reduced  rocks  and  rock-struct- 
ures forming  as  usual  the  most  prominent  features  in 
the  landscape. 

The  denudation  of  a  broad  anticline  composed  of 


86  EARTH  SCULPTURE 

harder  rocks  intercalated  among  a  series  of  more 
yielding  strata  results,  as  in  the  Wealden  area,  in  the 
formation  of  lines  of  escarpment  facing  each  other. 
In  the  case  of  a  denuded  syncline  of  similar  strata 
escarpments  are  likewise  developed,  but  their  faces 
are  now  turned  in  opposite  directions.  Fig.  2 1  shows 
the  geological  structure  of  a  portion  of  Ayrshire. 
Here  we  have  a  series  of  hard  volcanic  rocks  (V2),  old 
lavas,  in  fact,  intercalated  between  underlying  and 
overlying  sedimentary  strata — chiefly  sandstones  and 
shales.  The  result  is  the  same  as  in  all  the  cases 
already  considered — the  more  durable  rocks  crop  out 


FIG.  21.     SECTION  ACROSS  PERMIAN  VOLCANIC  BASIN,  AYRSHIRE. 

c ,  Carboniferous  strata  ;  v ,  volcanic  rocks  ;  /,  Permian  sandstones. 

strongly  and  form  escarpments,  but  these  look  away 
from  and  not  towards  each  other. 

In  regions  which  have  experienced  much  denuda- 
tion, gently  inclined  strata,  when  arranged  in  a  series 
of  anticlines  and  synclines,  not  infrequently  give  rise 
to  an  undulating  surface.  But  this  surface  does  not 
coincide  with  the  deformations  of  the  rocks  below. 
In  point  of  fact,  anticlines  are  not  infrequently  repre- 
sented at  the  surface  by  depressions,  and  synclines  by 
elevations.  These  phenomena  are  best  developed 
when  beds  or  masses  of  durable  nature  are  intercalated 
in  a  series  of  more  yielding  rocks.  In  the  accom- 


LAND-FORMS  IN  GENTL  Y  INCLINED  STRA  TA     87 

panying  section  (Fig.  22)  it  will  be  observed  that  syn- 
clines  coincide  with  hills,  and  anticlines  with  valleys. 
This  configuration  has  been  determined  by  the  geo- 
logical structure.  In  each  hill  we  have  practically  two 
escarpments  placed  back  to  back.  The  beds  h  h  are 
relatively  harder  than  others  in  the  series.  Had  no 
such  beds  occurred  the  synclines  would  probably  not 
have  been  so  strongly  emphasised  by  elevations.  But 
the  presence  of  one  or  more  hard  beds  in  series  of  un- 
dulating and  relatively  soft  strata  does  not  necessarily 
give  rise  to  synclinal  hills.  The  hard  beds  in  such  a 
series  would  no  doubt  in  time  crop  out  at  the  surface 


FIG.  22.     SYNCLINAL  HILLS  AND  ANTICLINAL  VALLEYS. 

s  s,  synclines  ;  a  a,  anticlines  ;  h  h,  relatively  hard  beds. 

and  project  above  the  base-level  of  the  district ;  but 
if  in  the  synclinal  troughs  they  descended  below  that 
level,  they  could  have  no  influence  upon  the  surface. 
Thus  in  the  section  (Fig.  23)  a  relatively  hard  bed 
crops  out  and  forms  escarpments  at  e  e,  but  it  descends 
below  the  base-level,  b  b,  in  the  two  synclinal  troughs 
(s1  s2),  which  remain  unaffected  by  it.  In  the  third 
trough  (^3),  however,  it  remains  above  the  base-level, 
protecting  the  underlying  softer  beds,  and  thus  forming 
a  hill. 


88 


EARTH  SCULPTURE 


When  a  series  of  undulating  strata  contains  no 
intercalated  hard  beds,  but  is  of  much  the  same 
consistency  throughout,  the  synclines  still  offer  the 
stoutest  resistance  to  denudation,  anticlines  being 
relatively  weak  structures.  In  the  former  the  strata 
are  not  liable  to  be  undermined  and  displaced  by  the 


FIG.  23.     ESCARPMENT  HILLS  AND  SYNCLINAL  HILL. 

e  £,  hard  bed ;  s1  s*  j3,  synclinal  troughs ;  b  b,  base-level. 

action  of  springs.  In  the  latter,  however,  the  strata 
hang  away  from  the  axis,  and  water  percolating 
through  them,  and  coming  out  along  the  bedding- 
planes,  tends  to  their  demolition.  But  this  is  a  mat- 
ter which  will  be  considered  more  fully  when  we  come 

Wttt  X«*»<nut 


FIG.  24.     SECTION  ACROSS  WEST  LOMOND  HILL  AND  THE  OCHILS. 

a,  igneous  rocks  ;  £,  red  sandstones,  etc. ;  c,  basalt. 

to  consider  the  surface-forms  yielded  by  steeply  in- 
clined and  highly  folded  strata. 

In  regions  long  exposed  to  denudation  all  weakly 
built  hills  tend  to  disappear.  Hence  in  such  countries 
anticlinal  hills  are  of  very  rare  occurrence.  Now  and 
again  they  do  occur,  but  only  when  they  happen  to  be 
composed  of  more  durable  rocks  than  those  which 


LAND-FORMS  IN  GENTL  Y  INCLINED  STRA  TA     89 

repose  upon  their  flanks.  The  Ochils  of  Kinross 
afford  us  a  good  example.  (Fig.  24.)  Here  we 
have  an  underlying  series  of  hard  igneous  rocks,  a, 
folded  along  an  axis  from  which  they  dip  away  on 
both  sides  below  overlying  sheets  of  red  sandstone. 
These  red  sandstones  almost  certainly  at  one  time 
extended  across  the  anticline,  which  has  thus  been 


FIG.  25.     SYNCLINAL  VALLEY  WEST  OF  GREEN  RIVER.     (Powell.) 

much  denuded.  But,  owing  to  the  greater  durability 
of  the  igneous  rocks,  the  anticline,  of  which  they 
form  the  axis,  continues  to  show  as  a  prominent 
elevation. 

Hitherto  we  have  been   considering  the   surface- 
forms    assumed  by  gently  folded    strata    in  regions 


9° 


EARTH  SCULPTURE 


which  have  been  subjected  for  a  more  or  less  pro- 
longed period  to  subaerial  denudation.  In  areas 
where  deformation  of  the  strata  has  been  effected 
within  geologically  recent  times,  not  infrequently 
some  coincidence  may  be  observed  between  the  un- 
dulations at  the  surface  and  the  underground  struct- 


FIG.  a6.    ANTICLINAL  RIDGE,  GREEN  RIVER  PLAINS.    (Powell.) 

ure.  The  Colorado  district  we  have  described  as  a 
region  of  practically  horizontal  strata.  Here  and  there, 
however,  the  rocks  are  more  or  less  folded,  and  when 
such  is  the  case  they  often  give  rise  to  corresponding 
folds  at  the  surface.  In  the  region  traversed  by 
Green  River,  for  example,  the  horizontal  strata  occa- 


LAND-FORMS  IN  GENTL  Y  INCLINED  STRA  TA     9 1 

sionally  show  anticlines  and  synclines,  as  in  the  follow- 
ing sketches  from  Major  Powell's  description  of  the 
Canon  country,  where  the  synclinally  arranged  beds 
in  Fig.  25  form  a  valley,  while  the  anticlinal  strata  iri 
Fig.  26  appear  as  a  swelling  ridge. 

Such  coincidence  of  underground  structure  and 
superficial  configuration,  however,  is  not  always  to  be 
traced  even  in  so  young  a  land  as  the  Canon  district, 
while,  as  already  remarked,  it  is  of  very  uncommon 
occurrence  in  lands  of  high  geological  antiquity. 


CHAPTER  V 

LAND-FORMS   IN   REGIONS  OF  HIGHLY  FOLDED 
AND  DISTURBED  STRA  TA 

TYPICAL    ROCK-STRUCTURES  IN  REGIONS  OF    MOUNTAIN-UPLIFT 

GENERAL  STRUCTURE  OF  MOUNTAINS  OF  UPHEAVAL — PRIMEVAL 
COINCIDENCE  OF  UNDERGROUND  STRUCTURE  AND  EXTERNAL 
CONFIGURATION — RELATIVELY  WEAK  AND  STRONG  STRUCT- 
URES—STAGES IN  THE  EROSION  OF  A  MOUNTAIN-CHAIN — 

FORMS    ASSUMED     UNDER    DENUDATION ULTIMATE     FATE    OF 

MOUNTAIN-CHAINS. 

WE  have  now  to  study  the  various  land-forms 
that  characterise  regions  where  highly  folded 
strata  occur.  Deformation  of  the  crust  has  taken 
place  in  all  ages  of  the  world's  history.  In  some 
countries  rock-plication  and  folding  date  back  to  the 
earliest  period  of  which  geologists  have  any  certain 
knowledge.  In  other  places  the  deformations  belong 
to  relatively  recent  times.  Again,  we  find  evidence 
to  show  that  certain  areas  have  experienced  such 
changes  at  many  successive  periods.  As  might  have 
been  expected,  the  oldest  rock-folds  have  suffered 
excessive  erosion,  while  the  youngest  have  experienced 
less.  We  are  thus  able  to  study  in  different  countries 
the  successive  phases  through  which  a  region  of  highly 

92 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     93 

disturbed  strata  must  necessarily  pass.  We  see  it  in 
its  youth  in  such  mountains  as  the  Alps,  the  Hima- 
layas, the  Cordilleras,  and  in  its  old  age  in  the 
Appalachians  and  the  mountains  of  Scandinavia  and 
Britain. 

Let  us  now  briefly  consider  some  of  the  typical 
kinds  of  structure  presented  by  the  more  steeply  in- 
clined strata.  In  regions  of  moderately  inclined  rocks 
the  folds,  as  we  have  seen,  are  symmetrical  anticlines 
and  synclines.  the  axes  of  which  are  vertical,  the  beds 


FIG.  27.     ISOCLINAL  FOLDS. 

Axes  moderately  inclined  from  the  vertical. 

dipping  away  from  or  towards  the  axes  at  approxi- 
mately equal  angles.  (See  Fig.  22,  p.  87.)  Folds  of  this 
kind,  however,  are  not  restricted  to  areas  of  moderately 
inclined  strata  ;  they  are  met  with  also  in  regions  where 
the  rocks  as  a  rule  dip  steeply.  But  in  such  regions 
the  anticlines  and  synclines  are  usually  more  or  less 
unsymmetrical — their  axes  are  inclined.  In  Fig.  27 
we  have  represented  a  series  of  moderately  inclined 
folds.  In  Fig.  28  the  inclination  of  the  axes  is  still 
greater.  As  the  folds  in  these  two  diagrams  all  lean 
in  one  direction,  they  are  said  to  be  isoclinal.  Very 
frequently  the  inclination  of  the  axes  increases  to  such 
a  degree  that  one  fold  may  come  to  lie  almost  hori- 


94  EARTH  SCULPTURE 

zontally  upon  another,  as  in  Fig.  29.     But  when  the 
axes  are  so  highly  inclined  as  that  the  folds  usually 


FIG.  28.     ISOCLINAL  FOLDS. 

Axes  much  inclined. 


tend  to  become  disrupted.  All  folds  are  the  result 
of  horizontal  push  or  tangential  pressure,  and  when 
this  is  very  great  they  may  yield  by  shearing,  and 


FIG.  29.     ISOCLINAL  FOLDS. 

Axes  horizontal  =  overfolds. 


one  limb  be  thrust  forward  over  the  other,  producing 
what  is  known  as  a  reversed  fault.     (Figs.  30,  31.) 

So  overpowering    has    been  the  horizontal  move- 
ment in  some  cases  that  masses  of  rock  thousands  of 


LAND-FORMS  IN  H1GHL  Y  FOLDED  STRA  TA     95 

feet  in  thickness  have  been  buckled  up  and  sheared, 
or,  simply  yielding  to  pressure,  have  sheared  without 
folding,  and  been  thrust  forward  for  miles  along  a 


FIG.  30.    OVERFOLD  PASSING  INTO  REVERSED  FAULT  OR  OVERTHRUST. 

gently  inclined  or  even  an  approximately  horizontal 
plane.  These  great  reversed  faults  are  termed  over- 
thrusts  or  thrust-planes.  Sometimes  such  thrust- 


FIG.  31.     REVERSED  FAULT. 

planes  occur  singly  (Figs.  32,  33),  at  other  times  the 
rocks  have  yielded  again  and  again,  great  sheets  hav- 
ing been  sliced  off  successively  and  driven  forward 
one  upon  the  other.  (Fig.  34.) 

Another  structure  encountered  in  regions  of  much 


FIG.  32.    SINGLE  THRUST-FLANK. 


96 


EARTH  SCULPTURE 


disturbed  strata  is  the  synclinal  double-fold,  shown  in 
the  annexed  diagram.  (Fig.  35.)  In  this  case  two 
anticlinal  folds  approach  each  other  from  different 
directions,  the  synclinal  depression  between  the 
approximating  anticlines  being  occupied  by  highly 
convoluted  strata. 

The  converse  of  this  structure  is  the  anticlinal  double- 
fold  as  shown  in  Fig.  36.     Here  two  synclinal  folds 

X 


FIG.  33.     SECTION  ACROSS  COAL-BASIN  OF  MONS.     (M.  Bertrand.) 

Z>1  />',  Lower  and  Upper  Devonian;    C7,  Carboniferous  Limestone;  O,  Cretaceous;  T, 

Overfold  and  thrust-plane.       Devonian  and  Carboniferous  strata  turned  upside 

down  above  the  thrust-plane. 

approach  each  other,  while  in  the  intervening  space 
the  strata  are  arched  into  a  great  anticline.  The  beds 
within  the  anticline,  it  will  be  observed,  are  much 
compressed  below,  while  they  open  out  above.  This 
is  known  as  fan-shaped  structure. 

Reverse  faults  and  thrust-planes  have  been  referred 
to,  but  it  must  be  noted  that  normal  faults  also  now 
and  again  occur  in  complicated  regions.  The  former, 
as  we  have  seen,  are  the  result  of  horizontal,  the  latter 
of  vertical  movements  of  the  crust.  Reversed  faults, 
therefore,  are  almost  entirely  restricted  to  regions 


I  \ 


98  EARTH  SCULPTURE 

where  the  rocks  are  more  or  less  steeply  inclined  and 
contorted.  Normal  faults,  on  the  other  hand,  occur 
under  all  conditions  of  rock-structure — traversing 
alike  horizontally  arranged  strata  and  inclined  and 
folded  beds  of  every  kind. 

So  much,  then,  for  the  general  types  of  structure 
met  with  among  highly  folded  strata.  So  far  as  our 
present  knowledge  goes,  complex  folding,  such  as  is 


FIG.  36.     ANTICLINAL  DOUBLE-FOLD. 

seen  in  true  mountains  of  uplift,  has  resulted  from 
horizontal  movement  in  one  direction.  This  is  shown 
by  the  manner  in  which  most  of  the  more  closely 
compressed  and  steeper  folds  of  a  mountain-chain 
tend  to  lean  over  one  way.  Under  the  influence  of 
an  irresistible  horizontal  thrust  the  strata  find  relief 
by  folding,  and  the  crust  bulges  upwards,  the  flexured 
rocks  naturally  bending  over  in  the  direction  of  least 
resistance.  The  resulting  structure  may  be  shown 
diagrammatically  as  in  Fig.  37.  In  this  diagram  only 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     99 

folds  are  represented  ;  in  many  cases,  however,  the 
rocks  are  not  merely  flexed,  folded,  and  contorted, 
but  dislocated  and  displaced.  Frequently,  indeed, 
they  have  yielded  to  the  intense  pressure  by  shearing, 
and  slice  after  slice,  hundreds  or  even  thousands  of 
feet  in  thickness,  has  been  pushed  forward  and  piled 
one  on  top  of  the  other.  Although  the  closer  folds 
tend  as  a  rule  to  lean  over  in  the  direction  of  crustal 
movement,  yet  occasionally  they  are  inclined  in  the 
opposite  direction,  thus  giving  rise  to  the  well-known 


FIG  37.     DIAGRAM  OF  MOUNTAIN  FLEXURES. 

The  arrow  shows  the  direction  of  thrust. 

fan-structure  seen  in  the  anticlinal  double-fold,  Fig. 
36.  Now  and  again,  too,  the  folds  may  open  out, 
and  so  form  symmetrical  flexures  with  vertical  axes, 
or  normal  anticlines  and  synclines.  The  cause  of 
such  variations  in  the  folding  of  the  strata  is  an  in- 
teresting question,  but  does  not  concern  us  here. 

When  a  tract  of  highly  disturbed  rocks  has  been  ex- 
posed to  erosion  for  a  very  prolonged  period,  it  is 
usually  hopeless  to  attempt  to  reconstruct  the  original 
configuration  of  the  ground,  save  in  a  very  general 
way.  The  primeval  land-forms  that  may  have  re- 
sulted from  crustal  deformation  have  been  entirely 
remodelled  or  removed  by  denudation.  But  there 


TOO  ^^     EARTH  SCULPTURE 

are  many  regions  where  similar  extensive  deformation 
has  taken  place  at  a  relatively  recent  geological  date, 
and  where,  therefore,  time  has  not  sufficed  for  the 
obliteration  of  all  surface-features  due  to  crustal  dis- 
turbance. In  the  younger  mountain-chains  of  the 
world,  underground  structure  and  orographical  fea- 
tures to  a  certain  extent  coincide.  The  study  of 
these  mountains,  therefore,  enables  us  to  realise  the 
conditions  that  formerly  obtained  in  tracts  of  highly 
complicated  structure,  from  which,  under  l 
tinued  erosion,  all  trace  of  the  original  configu 
of  the  ground  has  vanished.  Not  only  so,  but  the 
havoc  wrought  by  epigene  action  upon  even  the 
youngest  of  our  mountains  shows  us  how  and  by 
what  means  the  complicated  mountain-chains  of 
earlier  days  have  gradually  been  reduced.  For,  just 
as  lands  built  up  of  horizontal  and  gently  inclined 
strata  have  experienced  various  degrees  of  erosion, 
thus  enabling  us  to  trace  the  successive  stages  through 
which  such  lands  must  pass,  so  regions  of  highly  com- 
plex structure  present  us  with  various  phases  of  denud- 
ation. And  thus,  by  comparing  one  tract  with 
another,  we  may  spell  out  the  whole  story  ;  and  in 
the  degraded  relics  of  former  mountain-systems  we 
read  the  fate  that  must  eventually  overtake  the  proud- 
est elevations  of  the  present. 

The  study  of  the  land-forms  assumed  by  highly 
flexured  strata  should  naturally  begin  with  the  exam- 
ination of  some  young  mountain-chain.  But  even 
the  youngest  of  such  mountains  has  already  under- 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     101 

gone  much  erosion,  and  its  struSwre  is  often  ex- 
tremely complicated.  To  examine  any  one  system  in 
detail,  and  to  follow  the  whole  process  of  its  denuda- 
tion, would  be  a  laborious  work,  far  beyond  the  limits 
of  our  present  inquiry.  All  that  we  desire  is  to  ascer- 
tain if  we  can  how  far  geological  structure  and  oro- 
graphical  configuration  coincide  during  the  period  of 
a  mountain's  infancy  and  early  youth,  and  by  what 
means  its  original  form  becomes  modified  and  event- 

*  remodelled.  For  this  purpose  we  may  profitably 
our  study  by  considering  first  some  hypothetical 
We  shall  suppose,  then,  that  under  tangential 
pressure  the  horizontal  strata  of  some  region  have 
bulged  up  and  become  folded  along  a  given  line  or 
zone.  Under  such  conditions  great  faults  and  thrust- 
planes  would  be  likely  enough  to  occur  ;  but  for  the 
sake  of  simplicity  we  shall  ignore  these,  and  fix  our 
attention  only  on  the  flexing  and  folding.  We  shall 
suppose  further  that  our  mountain-chain  is  the  result 
of  one  prolonged  continuous  earth-movement.  How, 
then,  will  the  elevation  of  the  strata  affect  the  sur- 
face ?  Will  the  complex  folding  of  the  rocks  give 
rise  to  similar  intricate  deformations  of  the  surface  ? 
This  does  not  necessarily  follow,  for,  were  the  move- 
ment of  elevation  very  slow  and  protracted,  the  grad- 
ually rising  surface  might  be  so  continually  reduced 
by  denudation  that  underground  structure  and  exter- 
nal form  would  rarely  or  never  correspond.  But,  on 
the  other  hand,  were  the  rate  of  elevation  in  excess 
of  the  rate  of  erosion,  the  larger  folds  of  the  strata 


102  EARTH  SCULPTURE 

might  be  expected  to  give  rise  to  similar  undulations 
at  the  surface.  It  is  very  doubtful,  however,  whether 
the  latter  would  ever  be  as  strongly  pronounced  as 
the  former  ;  for  at  great  depths  the  folds  would  be 
pressed  closely  together,  while  they  would  naturally 
tend  to  open  out  upwards  into  broader  undulations. 
Hence,  deeply  buried  rock-masses  might  be  intensely 
flexed  and  folded,  while  the  surface  might  show  only 
a  more  or  less  pronounced  bulging.  The  infant 
mountain  might  appear  as  merely  one  single  long 
swell  or  undulation,  with  smooth  slopes,  declining  at 
no  great  angle  to  the  low  grounds.  Or  there  might 
be  a  series  of  two  or  more  such  undulations.  The 
study  of  existing  mountain-chains,  however,  leads  to 
the  belief  that  in  some  cases  at  least  very  considera- 
ble deformation  of  the  surface  has  accompanied 
mountain-making,  all  the  larger  folds  of  the  strata 
being  probably  at  first  represented  above  ground  by 
corresponding  ridges  and  depressions. 

We  do  not  know  whether  the  elevation  of  a  moun- 
tain-chain was  ever  suddenly  effected.  So  far  as  we 
can  judge  from  the  evidence  supplied  by  geological 
structure,  it  would  seem  as  if  the  horizontal  move- 
ments of  the  crust  had  been  gradual  and  protracted, 
and  often  interrupted  by  long  pauses.  There  is  little 
reason  to  doubt,  however,  that  during  the  growth  of 
a  mountain-chain  sudden  snapping  of  rocks  under 
pressure  must  have  occurred  frequently  enough,  and 
that  earthquakes  of  greater  or  less  intensity  must 
have  accompanied  the  upheaval.  If  such  has  been 


LAND-FORMS  IN  H1GHL  Y  FOLDED  STRA  TA     103 

the  case,  it  would  follow  that  the  surface  might  be 
very  considerably  affected — rocks  might  be  shattered 
and  weakly  constructed  ridges  shaken  down — so  that 
the  anticlinal  ridges  of  a  mountain-chain  might  well 
have  presented,  even  in  the  days  of  its  infancy,  a 
broken  and  ruptured  surface. 

But,  to  return  to  our  hypothetical  mountain-chain, 
we  shall  suppose  this  consists  of  a  series  of  parallel 
ridges  which  attain  their  greatest  elevation  along  a 
line  or  axis  not  far  removed  from  the  thrust-side  of 
the  chain.  From  this  axis  the  ridges  decline  gradually 
in  importance  in  the  direction  of  earth-movement, 
and  eventually  die  out  in  a  series  of  gentle  undula- 
tions. Each  of  the  ridges,  we  shall  suppose,  coin- 
cides with  an  anticline,  and  each  of  the  intervening 
hollows  with  a  syncline.  In  a  word,  we  shall  take 
the  surface  to  be  a  more  or  less  exact  expression  of 
the  geological  structure,  the  undulations  of  the  ground, 
however,  being  less  pronounced  than  those  of  the 
strata  at  considerable  depths.  The  diagram  (Fig.  37, 
page  99),  will  represent  a  section  across  such  a  chain. 
It  will  be  observed  that  all  faults  and  possible  intru- 
sions of  igneous  rock  are  neglected. 

In  any  series  of  stratified  rocks  some  are  sure  to 
be  more  porous  than  others,  while  all  will  be  traversed 
by  joints  or  cracks  approximately  at  right  angles  to 
the  bedding-places.  This,  then,  we  shall  take  to  be 
the  case  with  the  rocks  of  which  our  young  mountain- 
chain  is  composed  ;  and  we  shall  suppose  that  the 
parallel  ridges  extend  in  a  linear  direction  for  many 


io4  EARTH  SCULPTURE 

miles,  gradually  declining  in  elevation  towards  both 
ends  of  the  chain.  With  these  conditions  of  surface, 
it  is  obvious  that  drainage  will  take  place  in  the  di- 
rection of  the  great  longitudinal  valleys  or  synclinal 
troughs,  while  a  set  of  transverse  streams  will  flow 
down  the  slopes  of  the  anticlinal  ridges.  Many  of 
these  will  thus  become  tributary  to  the  rivers  making 
their  way  along  the  axial  hollows.  All  the  rivers  in 
course  of  time  must  cut  into  the  rocks,  but  it  is  obvi- 
ous that  the  transverse  streams  will  be  of  a  torrential 
character,  and  will  tend  therefore  to  carve  out  nar- 
rower, deeper,  and  straighter  channels  than  the  larger 
rivers  can  excavate  in  the  less  inclined,  broad  axial 
depressions.  Immense  quantities  of  rock-material 
will  be  swept  down  from  the  anticlinal  ridges  to 
accumulate  in  heaps  and  sheets  in  the  synclinal 
troughs,  or  to  be  swept  away  more  readily,  according 
as  the  gradients  of  the  latter  are  gentle  or  steep. 
Erosion,  in  short,  will  be  carried  on  most  actively 
upon  the  anticlinal  mountains.  This  would  naturally 
follow,  whatever  the  character  of  the  geological  struc- 
ture might  be,  for  the  erosive  action  of  running  water 
increases  with  the  gradient. 

But  in  all  cases  denudation  is  hastened  or  retarded 
according  as  the  rock-structure  is  weak  or  strong.  If, 
therefore,  the  mountains  of  our  hypothetical  chain  be 
more  weakly  built  than  the  parallel  synclinal  troughs, 
the  former  will  tend  to  be  reduced  more  rapidly  than 
the  latter.  This  can  be  shown  diagrammatically  as 
in  Fig.  38,  p.  105.  Here  we  have  a  section  across  two 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     105 

anticlinal  mountains  and  a  synclinal  valley.  The  strata 
consist  of  a  series  of  more  or  less  porous  sandstones 
separated  by  intervening  layers  of  impermeable  clay- 
rocks.  Moreover,  they  are  jointed,  and  the  joints 
traversing  the  anticlines  tend  to  open  out  upwards, 
while  the  reverse  is  the  case  with  those  cutting  the 
synclines.  Some  of  these  joints  may  be  shrinkage- 
cracks  which  came  into  existence  during  the  slow  con- 
solidation of  the  strata,  perhaps  long  before  the  latter 
were  flexed  and  folded.  But  a  large  proportion  no 
doubt  would  be  produced  while  the  rocks  were  being 
bent  and  doubled  up.  In  whatever  way  formed, 
joints  are  readily  permeated  by  meteoric  water,  which 
finds  its  way  down  from  the  surface  and  soaks  into 


FIG  38.     DIAGRAM  OF  ANTICLINAL  MOUNTAINS 

Pervious  strata  (stippled)  and  impervious  layers  (thin  lines)  ;  //,  joints,  cutting  strata  at  right 
angles  ;  v,  valley  ;  s  j,  springs  coming  out  at  junction  of  pervious  and  impervious  beds. 

the  porous  strata  below.  Constantly  augmented  from 
above,  the  water  thus  imbibed  is  forced  to  percolate 
through  the  porous  beds  in  the  direction  of  the  dip. 
Hence  wherever  these  beds  are  truncated  (as  in  the 
valley)  the  water  comes  out  at  the  surface  as  natural 
springs.  Thus  in  the  illustration  springs  appear  at 
s  s,  where  permeable  sandstones  are  underlaid  by  im- 


io6 


EARTH  SCULPTURE 


permeable  clay-rocks.  The  effect  of  these  springs  is 
not  hard  to  understand.  They  tend  to  undermine 
the  sandstones,  and  as  the  dip  of  the  strata  is  towards 
the  valley,  rock-falls  and  landslips  must  continue  to 
take  place  until  the  anticline  is  reduced.  Anticlinal 
mountains  separated  by  a  synclinal  trough  are  thus 
in  a  state  of  unstable  equilibrium.  Sapped  and  un- 
dermined by  rain,  frost,  and  springs,  their  existence 


FIG  39.    SYNCLINAL  VALLEY  SHIFTING  TOWARDS  ANTICLINAL  Axis. 

a,  synclinal  valley  ;  </,  anticline  ;  v,  valley,  gradually  widened  in  the  direction  of  the  arrow. 

cannot  be  prolonged.  On  the  other  hand,  the  strata 
in  the  synclinal  trough,  although  consisting  of  the 
same  materials,  will  be  relatively  more  durable.  Their 
arrangement  favours  their  preservation  ;  they  are  not 
sapped  and  undermined  as  in  an  anticline,  but  are 
reduced  chiefly  by  the  vertical  erosion  of  the  rivers 
that  traverse  them. 

The  anticlines  of  our  mountain-chain  are  thus  not 
only  deeply  incised  by  transverse  streams  and  torrents, 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     107 

but  they  are  liable  all  along  their  flanks  to  the  under- 
mining action  of  the  longitudinal  rivers  and  their  allies, 
—rain,  frost,  and  springs.  Quite  undisturbed  by 
earthquakes,  their  destruction  by  epigene  action  is, 
nevertheless,  assured.  But  if  the  young  mountain- 
chain  be  liable,  as  all  such  mountains  are,  to  earth- 
quake-shocks, the  demolition  of  the  already  weakened 
anticlines  will  often  be  greatly  accelerated. 

Unsymmetrical  anticlines  are  not  less  liable  to  de- 
struction than  those  we  have  just  been  considering. 
Indeed,  their  arrangement  must  lead  sometimes  to 
the  gradual  shifting  of  a  longitudinal  river  from  a 
synclinal  to  an  anticlinal  axis.  Thus  a  river  occupy- 
ing the  syncline  a  (Fig  39),  and  eventually  cutting 
more  or  less  deeply  into  the  underlying  strata,  will 
tend  to  work  its  way  towards  the  axis  of  the  anticline 
d.  For  it  will  be  observed  that  the  beds  of  that  anti- 
cline dip  into  the  valley,  while  those  on  the  other  side 
dip  away  from  it.  The  latter,  therefore,  is  a  strong 
structure,  and  the  valley-cliffs  will  recede  relatively 
slowly  in  that  direction,  while  rock-falls  and  landslips 
will  prevail  on  the  side  of  d.  The  valley,  therefore, 
will  be  widened  most  readily  towards  d ;  and,  the  like 
conditions  obtaining  in  all  the  longitudinal  valleys  of 
a  chain,  the  time  will  come  when  every  similarly  con- 
structed anticlinal  ridge  may  be  reduced. 

Many  other  modifications  of  the  drainage  of  a 
mountain-chain  will  be  brought  about  by  the  action 
of  the  streams  and  rivers.  Thus  a  transverse  stream, 
which  as  a  rule  works  more  energetically  than  a  longi- 


io8  EARTH  SCULPTURE 

tudinal  river,  may  now  and  again  succeed  in  cutting 
its  way  back  across  an  anticline  so  as  to  tap  some  ad- 
jacent synclinal  trough.  If  the  bottom  of  this  trough 
should  chance  to  be  at  a  higher  level  than  that  of  the 
hollow  into  which  the  transverse  stream  makes  its 
way,  the  river  of  the  invaded  syncline  may  be  cap- 
tured by  the  stream.  Thus  we  should  have  the  phe- 
nomenon of  a  longitudinal  river  changing  its  course 
and  becoming  transverse. 

The  chief  point,  however,  which  we  have  at  present 
to  bear  in  mind  is  simply  this  :  that  anticlinal  struc- 
tures are  weak  and  tend  to  be  reduced  ;  while  synclinal 
arrangements  are  relatively  strong,  and  consequently 
more  persistent.  We  should  expect  to  find,  therefore, 
in  all  mountains  of  upheaval,  exposed  for  any  time  to 
denudation,  that  synclinally  arranged  strata  will  not 
infrequently  appear  in  a  tolerable  state  of  preserva- 
tion ;  while  anticlinal  beds  will  often  be  deeply  eroded. 
Let  us,  then,  turn  our  attention  to  the  structures  met 
with  in  such  a  region  as  the  Alps,  and  see  how  far 
they  bear  out  these  elementary  conclusions. 

That  great  chain  is  a  typical  example  of  what  are 
known  as  mountains  of  elevation.  It  consists  essen- 
tially of  a  succession  of  anticlines  and  synclines, 
chiefly  unsymmetrical.  The  strata  are  not  only  folded 
and  often  exceedingly  contorted,  but  the  structure  is 
still  further  complicated  by  vast  thrust-planes  and 
normal  faults.  Moreover,  the  chain  is  the  result,  not 
of  one,  but  of  many  successive  earth-movements.  But 
the  chief  movement — that,  namely,  to  which  the 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     109 

mountains  owe  most  of  their  present  elevation — took 
place  at  a  relatively  late  geological  period.  Many  of 
the  folded  and  fractured  rocks,  indeed,  are  of  no 
greater  antiquity  than  the  soft  clays  and  sands  over 
which  London  is  built.  And  yet,  although  the  chain 
belongs  to  so  late  a  date,  its  rocks  everywhere  bear 
witness  to  great  erosion.  Enormous  masses  of  ma- 
terial have  been  gradually  removed,  and  the  original 
surface,  due  to  folding  and  displacement,  has  been 
more  or  less  profoundly  modified. 

The  sketch-section  across  the  Swiss  Alps  (Fig.  40, 
p.  no)  gives  the  general  arrangement  of  the  strata, 
and  enables  us  in  some  faint  measure  to  appreciate 
the  degree  of  denudation  which  has  already  been  ex- 
perienced by  these  relatively  young  mountains. 
Grant,  if  you  will,  that  the  folding  of  the  strata  may 
have  resulted  in  a  kind  of  chaos  at  the  surface — that 
the  ground  along  the  axes  of  anticlinal  arches  may 
have  been  ruptured,  and  the  rocks  everywhere  tum- 
bled in  confusion — yet  we  have  still  to  account  for 
the  wholesale  removal  of  the  abundant  debris — the 
shattered  reefs  and  dislodged  mountain-masses.  We 
cannot,  in  short,  escape  from  the  conclusion  that  an 
enormous  amount  of  denudation  has  taken  place. 
So  profoundly  has  the  original  configuration  been 
modified,  that  it  is  only  when  the  mountains  are 
viewed  in  the  broadest  way  that  any  coincidence  be- 
tween underground  structure  and  surface-features 
can  be  observed.  Even  where  anticlines  still  form 
hills  and  mountains  it  is  obvious  that  they  have  yet 


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LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     in 

suffered  extensive  degradation.     (See  Fig.  41.)    Not 
infrequently,  indeed,   they  are  more  or   less   deeply 


FIG.  41.     SUMMIT  OF  SANTIS,  EAST  SIDE  (A.  Heim). 

Anticlinal  mountain. 

trenched — valleys  running  along  their  axes,  an  ap- 
pearance well  shown   in  Fig.  42.     Synclinal  hollows 


FIG.  42.     SECTION  ACROSS  THE  SCHORTENKOPF,  BAVARIAN  ALPS 

(E.  Fraas). 
Anticlinal  valley  in  calcareous  rocks  and  shales  (Triassic.) 


112 


EARTH  SCULPTURE 


now  and  again  coincide  with  depressions  at  the  sur- 
face, as  in  Fig.  43  ;  but  they  just  as  often,  or  even 


FIG.  43.     SECTION  ACROSS  THE  KAISERGEBIRGE,  EASTERN  ALPS  (E.  Fraas). 

Synclinal  valley  in  calcareous  rocks  and  shales  CT  riassic). 


FIG.  44.     SECTION  ACROSS  THE  VAL  D'UINA  (Gumbel). 

Triassic  strata  resting  on  crystalline  schists. 


FIG.  45.      SlCHELKAMM   OF    WALLENSTADT   (Helm). 
Sickle-shaped  overfold. 


LA^D-FORMS  IN  HIGHLY  FOLDED  STRATA     113 

more  frequently,  form  elevations,  as  in  Figs.  44,  45. 
In  every  case,  however,  the   evidence  of  denudation 


H^.'/tff\f,t'  v  V-», 

FIG.  46.     SECTION  ACROSS  THE  NORTHERN  LIMESTONE  ALPS  (E.  Fraas). 

7,  Crystalline  schists  ;  2,  Permian  ;  j,  Bunter  ;  4,  Muschelkalk  ;  5,  Limestone  (Wetterstein- 
kalk) ;  d,  Dolomite  ;  7,  Jurassic  and  Cretaceous. 

is  conspicuous.     Nor  is  this  less  clearly  seen  in  the 
more  complicated  structures  of  the  Alps.     In  the  fol- 


FIG.  47.     SECTION  ACROSS  THE  DIABLERETS  (Renevier). 

Tertiary  strata  showing  a  succession  of  overfolds. 

lowing  section,  for  example  (Fig.  46),  we  have  a  series 
of  various  calcareous  strata  and  underlying  schists 
compressed  into  folds  and  dislocated,  the  tops  of  the 


EARTH  SCULPTURE 


anticlines  having  in  each  case  been  removed.  Take 
again  the  section  of  the  Diablerets  (Fig.  47),  in  which 
the  Tertiary  strata  are  doubled  back  upon  themselves 


FIG.  48.      SECTION  ACROSS  DENT  DE  MORCLES  (Renevier). 

/,  Schistose  rocks,  etc.  ;  2,  Carboniferous  strata  ;  J.  Jurassic  strata  ;  4,  Cretaceous  strata  ;  5, 
Tertiary  strata  ;  #,  £",  y£,  Cretaceous  and  Tertiary  rocks  inverted  ;  7",  thrust-plane. 

in  a  series  of  sharp  overturned  flexures.  A  similar, 
but  somewhat  more  complicated,  structure  appears  in 
the  Dent  de  Morcles  (Fig.  48),  where  the  remarkable 


^ 

FIG.  49.     INVERSION  AND  OVERTHRUST  IN  THE  MOUNTAINS  SOUTH  OF  THE 
LAKE  OF  WALLENSTADT  (E.  Fraas,  after  A.  Heim). 

j,  Schistose  rocks  ;  /,  Permian  ;  wj\  bj\  Jurassic  ;  c,  Cretaceous  ;  ^,  Eocene.     The  Permian 
strata  (/)  are  turned  upside-down  and  thrust  upward  over  the  contorted  Eocene  (e). 

overturn  flexure  rests  upon  a  thrust-plane.  Here, 
again,  the  strata,  it  will  be  observed,  are  doubled  back 
upon  themselves,  or  turned  upside-down.  Obviously 
these  mountains  are  monuments  of  excessive  erosion. 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     115 

Similar  evidence  of  vast  rock-removal  is  furnished  by 
the  remarkable  double-folds  and  overthrusts  in  the 
mountains  of  the  Cantons  Glarus  and  St.  Gall,  as 
described  by  Heim  and  others  (See  Fig.  49.) 

Similar  conclusions  may  be  drawn  from  the  appear- 
ances presented  by  every  kind  of  rock-structure 
throughout  the  whole  extent  of  the  Alps. 

In  the  Jura  mountains  the  rock-foldings  are  some- 
times symmetrical,  and  anticlines  and  synclines  now 
and  again  coincide  with  hills  and  valleys  respectively, 
as  in  Fig.  50. 

It  will  be  observed,  however,  that  the  synclinal 
strata  have  suffered  less  erosion  than  the  intervening 


FIG.  50.    SYMMETRICAL  FLEXURES  OF  THE  JURA  MOUNTAINS. 

i 

Anticlinal  mountains  and  synclinal  valleys. 

anticlinal  strata.  In  the  western  part  of  the  same 
range  of  mountains  the  folds  are  less  symmetrical, 
but  they  yield  the  same  evidence  of  denudation.  The 
accompanying  section  (Fig.  51,  p.  1 16)  shows,  indeed, 
that  the  saddlebacks  have  not  only  been  considerably 
reduced,  but  are  even  beginning  to  develop  into  val- 
leys ;  while  the  synclines,  on  the  other  hand,  have 
experienced  less  erosion,  those  with  approximately 
vertical  axes  appearing  as  dominant  heights. 

Excellent  examples   of    the  same  phenomena  are 
furnished  by  the  Carpathians — a  mountain-chain  also 


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116 


LAND-FORMS  IN  HIGHLY  FOLDED  STRA  TA     117 


of  relatively  recent  age.  Fig.  52  £ 
(p.  1 1 6)  exhibits  the  structure  of 
a  part  of  the  chain  in  which  the 
folds  are  unsymmetrical.  Here 
it  will  be  observed  that  the  tops  of 
the  anticlines  have  in  every  case 
been  greatly  reduced  ;  but  the 
synclines,  owing  to  the  isoclinal  -: 
arrangements  of  the  strata,  do  ^ 
not  tend  to  develop  into  hills.  j 
In  point  of  fact,  unsymmetrically 
folded  strata  behave  very  much 
in  the  same  way  as  beds  having 
a  persistent  dip  in  one  direction. 
When  the  anticlines  have  been 
truncated  the  strata  appear  at 
the  surface  as  a  series  of  isoclinal 
beds,  some  of  which  are  rela- 
tively more  resistant  than  others. 
In  time,  therefore,  these  harder 
beds  crop  out  as  well-marked 
ridges  or  escarpments,  according 
as  the  angle  of  dip  is  high  or  rela- 
tively low.  But  no  sooner  do 
the  axes  of  the  folds  approach  the 
vertical,  and  the  flexures  become 
symmetrical,  than  the  superior 
strength  of  the  synclinal  structure  | 
at  once  asserts  itself.  This  is  well 
illustrated  by  Fig.  53,  where  we  have  a  series 


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EARTH  SCULPTURE 


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clinal  troughs  forming  conspicuous  mount- 
ains, while  the  intermediate  anticlines  cor- 
respond for  the  most  part  with  valleys 
and  depressions. 

If  it  be  true,  therefore,  that  the  denuda- 
tion of  young  mountains,  such  as  the  Alps 
and  the  Carpathians,  has  been  guided  and 
determined  to  a  large  extent  by  geological 
structure,  we  ought  to  meet  with  still 
stronger  evidence  of  a  like  kind  in  mount- 
ain-ranges of  greater  antiquity.  The 
mountain-systems  we  have  been  consider- 
ing are  of  Csenozoic  age ;  they  are  among 
the  latest  great  upheavals  of  the  world. 
We  see  in  the  Appalachian  Chain  of  North 
America  a  very  much  older  system,  for  it 
came  into  existence  about  the  close  of 
Palaeozoic  times.  Being  of  such  enormous 
antiquity,  the  Appalachians  ought  to  give 
evidence  of  correspondingly  great  denuda- 
tion. All  the  weak  geological  structures 
should  have  collapsed  and  disappeared 
ages  ago  ;  the  heights  ought  not  to  coin- 
cide with  anticlines.  The  accompanying 
section  across  a  portion  of  the  chain  in 
Pennsylvania  shows  that  this  has  actually 
happened,  symmetrical  synclines  having  as 
usual  developed  into  hills,  while  anticlines 
have  been  degraded. 

Similar  evidence  might  be  adduced  from 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     119 

many  other  regions,  but  enough  has  been  advanced 
to  show  that  in  the  process  of  erosion  and  denudation 
of  mountains  of  uplift,  anticlines,  as  compared  with 
synclines,  are  essentially  weak  structures.  When  the 
flexures  are  symmetrical  the  synclines  tend  to  be 
carved  into  hills,  but  when  the  axes  are  inclined  the 
strata  often  give  rise  to  a  series  of  prominent  escarp- 
ments or  to  a  succession  of  ridges  with  intervening 
hollows,  the  escarpments  and  ridges  corresponding  to 
the  outcrops  of  the  more  resistant  rocks.  (Fig.  55.) 
Comparing  mountain-chain  with  mountain-chain, 
we  find,  as  might  have  been  expected,  that  the  oldest 
mountains,  if  they  are  the  least  prominent,  are  at  the 
same  time  the  most  stable.  They  have  endured  so 
long  that  much  of  their  primeval  elevation  has  been 
lost ;  the  weakly  built  structures  have  been  demolished, 
and  only  the  stronger  now  remain.  Great  rock-falls 
and  landslips  are  therefore  seldom  heard  of  among 
such  mountains.  It  is  quite  otherwise  with  the 
younger  uplifts  of  the  globe.  The  valleys  of  the 
Alps,  the  Caucasus,  the  Himalayas,  the  Cordilleras, 
and  other  chains  of  relatively  recent  age  are  cumbered 
with  chaotic  heaps  of  fallen  rock-masses.  From  time 
to  time  peaks  and  whole  mountain-sides  collapse  and 
slide  into  the  valleys  ;  and  this  rapid  degradation  will 
continue  until  every  weak  structure  has  been  removed. 
The  hills  and  mountains  of  our  own  country  have  long 
since  passed  through  this  phase  of  unstable  equilib- 
rium. In  the  younger  mountain-chains  of  the  globe 
underground  structure  and  superficial  configuration 


120  EARTH  SCULPTURE 

still  to  a  certain  extent  coincide,  but  in  the  more 
ancient  and  therefore  more  highly  denuded  mountain- 
systems  such  coincidence  is  of  very  rare  occurrence. 
Anticlinal  mountains  built  up  of  porous  and  relatively 
impermeable  strata  are  restricted  to  regions  of  recent 
uplift,  and  have  no  long  life  before  them. 

We  have  seen  that  in  the  case  of  plains  and  plateaux 
of  accumulation  the  original  surface  of  the  ground  is 
an  expression  of  the  geological  structure,  the  general 
direction  of  their  drainage-systems  being  determined 


FIG.  55.  UNSYMMETRICAL  FOLDS,  GIVING  RISE  TO  ESCARPMENTS  AND  RIDGES. 

h  h,  hard  beds  ;  J  s,  soft  beds. 

by  the  average  inclination  of  the  strata.  The  same 
is  no  doubt  to  a  large  extent  true  of  regions  of  mount- 
ainous uplift ;  the  shape  of  the  surface  and  the 
direction  of  the  streams  and  rivers  must  at  first  have 
been  determined  by  the  arrangement  or  architecture 
of  the  rocks.  But  while  it  is  comparatively  easy  to 
realise  the  conditions  that  obtained  in  a  plateau-coun- 
try during  the  early  stages  of  its  existence,  it  is  very 
much  harder  to  picture  to  ourselves  the  general  aspect 
which  a  mountain-chain  must  have  presented  at  the 
time  of  its  upheaval.  We  are  justified  by  the  evidence 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     121 

in  believing  that  the  larger  inequalities  of  the  surface 
must  often  have  coincided  with  corresponding  flexures 
and  other  deformations  of  the  strata.  But  we  need 
not  suppose  that  all  the  convolutions,  fractures,  and 
displacements  now  laid  bare  in  precipice  and  gorge 
actually  appeared  as  such  at  the  surface.  Laboratory 
experiments  have  shown  that  a  great  deal  of  flexing, 
folding,  contortion,  and  displacement  may  take  place 
underground,  while  the  surface  simply  swells  up  or 
bulges.  And  that  may  quite  well  have  been  the  case 
with  many  mountain-chains.  Yet  we  cannot  ignore 
the  possibility  or  probability  that  folding  and  displace- 
ment of  strata  may  sometimes  have  resulted  in  whole- 
sale rupture  and  confusion  at  the  surface.  We  need 
not  wonder,  therefore,  if  we  sometimes  find  it  hard  to 
account  for  certain  vagaries  in  the  drainage-systems 
of  mountain-chains.  Even  the  youngest  of  these 
chains  has  experienced  so  much  denudation,  that  it 
is  often  impossible  to  realise  the  surface-conditions 
which  may  have  determined  the  initial  directions  of 
the  rivers.  The  longitudinal  watercourses  doubtless 
follow  the  axial  arrangement  of  the  strata,  some  of 
them  occupying  structural  hollows  (synclines),  while 
others  run  along  the  backs  of  anticlines,  or  follow  the 
outcrops  of  relatively  softer  rocks.  The  origin  of 
certain  transverse  river-courses  is  harder  to  under- 
stand. Some  of  these  may  cut  across  a  succession  of 
great  ridges  ;  they  break  through  the  mountains  in 
such  a  way  as  to  suggest  that  they  are  perhaps  follow- 
ing a  line  of  fracture.  Most  commonly,  however,  this 


122  EARTH  SCULPTURE 

is  certainly  not  the  case.  Sometimes  it  can  be  shown, 
as  already  indicated,  that  a  transverse  stream  has 
simply  eaten  its  way  back  into  the  heart  of  the 
mountain-ridge,  which  it  has  eventually  breached  or 
11  gapped,"  and  so  worn  down  as  to  encroach  upon  the 
drainage-area  of  some  adjacent  longitudinal  valley. 
Transverse  streams  working  back  in  this  way  have 
not  infrequently  captured  longitudinal  rivers,  which 
thus  appear  to  mysteriously  forsake  their  own  valley 
in  order  to  break  through  a  mountain-ridge.  Perhaps 
most  of  the  sudden  changes  in  direction  of  Alpine 
rivers  are  illustrations  of  this  system  of  capture.  It 
is  possible,  however,  as  some  geologists  have  sup- 
posed, that  certain  transverse  river-courses  may  have 
been  determined  by  the  presence  of  a  series  of  minor 
crustal  folds,  arranged  at  right  angles  to  the  main  or 
longitudinal  flexures  of  a  mountain-chain.  But  we 
know  so  little  of  the  actual  conditions  of  surface  that 
obtained  when  such  a  chain  was  being  upheaved,  that 
we  must  often  be  content  to  remain  in  ignorance  of 
the  causes  that  may  have  led  to  the  sudden  deflection 
of  a  river  across  a  mountain-ridge.  When  we  bear  in 
mind,  however,  that  the  present  lines  of  drainage  can 
agree  only  in  a  general  way  with  those  that  came  into 
existence  at  the  birth  of  a  chain — that  many  anticlinal 
arches,  now  laid  bare  and  deeply  eroded,  may  never 
have  shown  at  the  original  surface — it  is  not  hard  to 
understand  how  certain  transverse  river-courses  may 
have  come  to  intersect  a  succession  of  ridges.  In 
many  cases  such  courses  may  really  indicate  the 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     123 

primeval  inclination  of  the  ground,  the  rivers  having 
cut  their  way  at  first  without  any  reference  to  deeply 
buried  structures,  which  were  only  to  be  exposed 
later  on  during  the  general  process  of  denudation. 

Although  we  may  vainly  endeavour  to  trace  the 
history  of  all  the  river-courses  of  a  mountain-chain, 
we  need  be  in  no  doubt  as  to  the  ultimate  fate  of  the 
mountains  themselves.  It  is  more  difficult  certainly 
to  discover  the  various  stages  in  the  erosion  of  a 
mountain-system  than  in  that  of  a  plateau  of  accu- 
mulation ;  but  we  are  assured  that  all  elevated  lands, 
whatsoever  their  origin,  tend  to  be  lowered  to  their 
base-level.  Should  that  base-level  be  steadfastly 
maintained,  mountains  and  plateaux  alike  must  event- 
ually be  reduced  to  the  condition  of  plains  of  erosion. 
But  the  modifications  of  the  surface  of  a  mountain- 
region  developed  during  the  process  of  erosion  are 
infinitely  complex.  This  is  due  partly  to  the  very 
varied  composition  of  the  rocks,  and  partly  to  the 
complicated  geological  structure. 

The  surface-features  of  a  denuded  plateau  of  accu- 
mulation have  a  general  sameness ;  there  is  little 
variety  in  the  form  of  the  hills  and  mountains — all 
are  more  or  less  pyramidal.  In  regions  of  gently 
inclined  and  undulating  strata  the  features  due  to 
erosion  are  more  diversified,  and  this  diversity  be- 
comes greater  as  the  dips  of  the  strata  increase  and 
change  rapidly  in  direction.  The  foothills  that  flank 
the  base  of  so  many  mountains  of  uplift  are  com- 
posed very  often  of  symmetrically  folded  strata,  but  as 


i24  EARTH  SCULPTURE 

we  pass  inwards  to  the  main  chain  the  folds  become 
steeper  and  unsymmetrical,  and  the  structure  is 
rendered  still  more  complex  by  vast  overthrusts 
and  shearing-planes.  As  the  structural  complexity 
increases,  and  the  rocks  are  thrown  and  twisted  into 
every  possible  position,  the  surface-features  are  con- 
stantly, changing,  so  as  to  show,  often  within  narrow 
limits,  every  variety  of  cliff  and  ridge  and  peak.  We 
see  then  that  it  is  geological  structure  chiefly  that 
determines  the  form  of  the  ground  ;  and  since  the 
inclination,  the  folding,  and  the  shearing  of  rocks 
must  be  attributed  to  crustal  movement,  it  is  clear 
that  hypogene  action  has  played  a  most  important 
part  in  the  formation  of  mountains.  We  may  say  with 
truth  that  all  true  mountain-ranges  owe  their  origin 
to  deformation  of  the  crust.  But  the  shape  which 
they  ultimately  assume  is  solely  the  result  of  erosion. 
It  is  hypogene  action  which  provides  the  rough  blocks  ; 
it  is  by  epigene  action  that  these  are  subsequently 
carved  and  chiselled,  the  forms  of  the  sculptured 
masses  being  determined  by  the  nature  and  structure 
of  their  materials.  In  regions  of  recent  uplift,  the  pro- 
cess of  sculpturing,  although  considerably  advanced, 
has  not  yet  sufficed  to  obliterate  the  original  or 
primeval  shape  of  all  the  masses.  But  in  elevated 
tracts  of  great  antiquity  the  land-blocks  have  been 
entirely  remodelled.  In  the  general  lowering  of  the 
surface  by  denudation,  mountain-masses  have  been 
removed,  and  what  were  formerly  depressed  areas 
now  often  appear  as  dominant  elevations.  Mountains 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     125 

of  recent  uplift  are  characterised  by  steep  profiles, 
by  peaks  and  knife-edged  aretes  ;  the  structures  are 
often  unstable,  and  yield  readily  to  the  agents  of 
erosion,  so  that  rock-falls  and  landslips  are  constantly 
taking  place.  In  regions  of  ancient  uplift,  on  the 
other  hand,  the  profiles  are  generally  softer;  peaks 
and  sharp-crested  ridges  are  of  less  frequent  occur- 
rence, weak  structures  have  disappeared,  and  the 
degradation  of  the  mountains  does  not  advance  so 
rapidly.  The  levelling  process,  however,  though 
slower,  is  quite  apparent.  The  valleys  are  widened 
and  deepened,  the  mountains  crumble  down,  and, 
should  the  base-level  of  erosion  be  retained,  the 
whole  area  will  eventually  be  flattened  out  and 
resolved  into  a  plain  of  erosion. 

Such  then  are  the  several  stages  through  which  a 
region  of  mountain-uplift  must  pass.  First  comes  the 
stage  of  youth,  when  the  surface  configuration  corre- 
sponds more  or  less  closely  with  the  underground 
structure.  Next  succeeds  the  stage  of  middle-life, 
when  such  coincidence  is  all  but  obliterated,  when  the 
valleys  of  youth  have  been  exalted  and  its  mountains 
have  been  laid  low.  Last  comes  old  age  and  final 
dissolution,  when  the  whole  region  has  been  reduced 
to  its  base-level.  But  the  decay  of  a  mountain-chain 
does  not  always  proceed  without  interruption.  Not 
infrequently  the  base-level  is  disturbed  ;  new  hori- 
zontal movements  of  the  crust  take  place,  and  bulging- 
up  of  the  region  is  accompanied  by  further  folding 
and  fracturing  of  the  strata.  The  mountain-system 


126  EARTH   SCULPTURE 

renews  its  youth.  On  the  other  hand,  the  old  base- 
level  may  be  destroyed  by  subsidence  of  the  crust, 
and  the  mountains,  partially  or  wholly  drowned,  may 
in  time  become  largely  buried  under  new  accumula- 
tions of  sediment.  Re-elevation  taking  place,  erosion 
recommences,  and  the  degradation  of  the  region  is 
resumed.  In  the  structure  of  not  a  few  mountain- 


FIG.  56.     STRUCTURE  OF  THE  ARDENNES  (after  Cornet  and  Briart). 

MM,  the  existing  surface ;  the  light-shaded  area  above  this  level  represents  the  rock-masses 
removed  by  denudation.  The  Silurian  rocks  at  the  base  of  the  section  are  indicated  by  thin 
white  lines.  Above  these,  on  the  left-hand  side  of  the  section,  between  C  and  M^  come 
Devonian  conglomerate,  sandstone,  shale,  and  limestone ;  next  in  succession  follow  the 
Carboniferous  strata  at  and  above  M ;  A  A^  B  B,  C  C,  are  dislocations. 

chains  we  may  read  the  history  of  many  such  vicissi- 
tudes. 

So  completely  have  some  mountains  been  removed 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     127 

by  denudation,  that  without  some  knowledge  of  geo- 
logical structure  we  should  never  have  divined  their 
former  existence.  An  instructive  example  is  furn- 
ished by  the  Carboniferous  tracts  of  Belgium  and 
Northern  France.  The  structure  of  these  regions 
shows  that  formerly  a  considerable  range  of  moun- 
tains extended  between  Boulogne  and  Aix-la-Cha- 
pelle.  At  or  towards  the  close  of  Carboniferous  times 
a  great  earth-movement,  acting  in  a  direction  from 
south  to  north,  buckled  up  the  strata,  and  these, 
yielding  to  the  pressure,  snapped  across,  and  exten- 
sive overthrusting  followed  along  the  line  referred  to, 
the  Carboniferous  beds  being  inverted  and  overlaid 
by  Devonian  strata.  The  mountains  of  upheaval 
which  thus  came  into  existence  attained  a  great 
elevation,  the  higher  parts  of  the  range  reaching 
probably  not  less  than  16,000  or  18,000  feet.  The 
section  (Fig.  56)  will  show  how  completely  the  sur- 
face has  been  remodelled,  how  mountains  of  elevation 
have  been  replaced  by  a  plain  of  erosion. 


CHAPTER  VI 

LAND-FORMS  IN  REGIONS  OF  HIGHLY  FOLDED 
AND  DISTURBED  STRATA  (continued} 

STRUCTURE  AND  CONFIGURATION  OF  PLATEAUX  OF  EROSION — 
FORMS  ASSUMED  UNDER  DENUDATION — MOUNTAINS  OF  CIR- 
CUMDENUDATION HISTORY  OF  CERTAIN  PLATEAUX  OF  ERO- 
SION  SOUTHERN  UPLANDS  AND  NORTHERN  HIGHLANDS  OF 

SCOTLAND — STAGES   IN    EROSION    OF    TABLE-LANDS. 

IN  our  last  chapter  we  considered  the  history  of  a 
mountain-chain,  following  that  history  from  the 
stage  of  youth  to  old  age  and  final  dissolution.  This 
last  we  recognised  in  the  plain  of  erosion.  We  have 
next  to  trace  the  subsequent  history  of  such  a  plain. 
The  geological  structure  of  many  mountain-chains,  as 
already  indicated,  reveals  the  fact  that  these  are  often 
the  result  of  more  than  one  uplift.  After  having  been 
for  long  ages  subjected  to  erosion,  and  even  to  sub- 
sequent subsidence  and  sedimentation,  the  same  region 
has  again  yielded  to  lateral  crush,  and  new  series  of 
folds  and  thrust-planes  have  come  into  existence.  But 
the  crust  does  not  always  yield  in  this  particular 
fashion.  Not  infrequently  relief  from  pressure  is  ob- 
tained by  widespread  bulging-up  of  the  surface,  one 
or  more  broad  swellings  with  perhaps  corresponding 

128 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     129 

broad  depressions  appear,  instead  of  an  intricate  ar- 
rangement of  more  or  less  closely  compressed  folds. 
We  may  for  convenience'  sake  speak  of  the  latter  as 
resulting  from  axial  uplift,  and  of  the  former  as  due 
to  regional  uplift,  even  although  it  be  obvious  that  in 
most  wide  regions  of  uplift  there  must  be  an  axis  or 
line  of  maximum  movement. 

Now  it  can  be  shown  that  one  and  the  same  region 
has  not  infrequently  experienced  both  kinds  of  uplift. 
Axial  uplifts  have  in  time  been  succeeded  by  regional 
uplifts ;  for  again  and  again  we  encounter  ancient 


FIG  57.     DIAGRAMMATIC  SECTION  ACROSS  A  PLATEAU  OF  EROSION. 

Isoclinal  folds. 

plains  of  erosion  occurring  at  various  levels  above 
the  sea,  their  geological  structure  showing  clearly 
that  they  have  replaced  old  mountains  of  complicated 
structure.  Such  elevated  plains  may  be  termed  plat- 
eaux or  table-lands  of  erosion,  to  distinguish  them 
from  plateaux  of  accumulation  or  deposit.  The 
characteristic  feature  of  the  latter,  it  will  be  remem- 
bered, is  the  general  coincidence  of  the  surface  with 
the  underground  structure,  while  the  former  shows 
no  such  correspondence.  The  structure  of  a  table- 
land of  erosion  may  thus  be  represented  as  in  Fig  57. 
Many  such  table-lands  are  recognised  in  Europe, 
the  Highlands  and  Southern  Uplands  of  Scotland 


130  EARTH  SCULPTURE 

and  the  Scandinavian  plateaux  being  good  examples. 
Ancient  plateaux  of  the  kind  are  all  more  or  less  de- 
nuded, trenched,  and  furrowed  by  valleys  to  such  an 
extent  that  the  plateau  character  is  often  somewhat 
obscured.  For  no  sooner  is  a  plain  of  erosion  up- 
lifted than  a  new  cycle  of  erosion  begins.  The  di- 
rection of  the  drainage  is  determined,  in  the  first 
place,  by  the  slope  of  the  ground,  and  this  we  can 
readily  understand  may  be  somewhat  diversified.  The 
surface  may  be  canted  either  in  one  direction  only, 
or  in  more  than  one,  for  the  crustal  movement  is  un- 
likely to  be  equal  in  amount  throughout  the  whole 
region  of  uplift.  Hence,  the  primeval  rivers  may  all 
flow  in  one  particular  direction,  or  they  may  trend  to 
various  points  of  the  compass.  However  that  may 
be,  it  is  certain  that  in  course  of  time  they  must 
gradually  deepen  their  valleys,  and  the  plateau  must 
eventually  come  to  be  cut  up  very  much  in  the  same 
way  as  a  plateau  of  accumulation.  But  the  mountains 
of  circumdenudation  resulting  from  this  process  will 
differ  considerably  in  character  from  those  carved  out 
of  horizontal  strata.  The  varying  structure  of  the 
rocks  will  necessarily  influence  erosion,  and  thus  lead 
to  a  greater  diversity  of  form.  Should  the  strata  be 
steeply  inclined,  and  this  will  usually  be  the  case,  then 
it  is  obvious  that  the  harder  masses  must  come  in 
time  to  project  beyond  the  more  readily  reduced 
rocks  with  which  they  are  associated.  The  general 
surface  of  the  plateau  will  thus  tend  to  assume  a  cor- 
duroy configuration,  the  long  ridges  coinciding  with 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     131 

the  outcrops  of  the  "  harder  rocks,"  while  the  inter- 
vening parallel  hollows  will  correspond  with  the  out- 
crops of  the  more  yielding  strata.  In  short,  the 
land-features  evolved  by  denudation  will  have  a  gen- 
eral resemblance  to  those  produced  in  a  region  of 
slightly  inclined  and  gently  undulating  formations. 
But  owing  to  the  very  varied  character  of  the  rocks; 
and  their  more  complicated  structures,  the  surface- 
features  of  a  plateau  of  erosion  will  be  more  pro- 
nounced and  much  more  irregular.  In  such  a  region 
the  larger  rivers,  being  frequently  of  primeval  origin, 
will  often  be  found  to  cut  across  mountain-ridge  after 
mountain-ridge,  and  to  follow  courses  more  or  less 
transverse  to  the  corduroy  surface.  Others  may  keep 
closely  to  the  outcrops,  and  run  in  the  direction  of 
the  "strike"  or  trend  of  the  strata,  while  some  may 
take  now  one  route  and  now  another.  The  original 
surface  of  the  plateau  will  generally  be  indicated  by 
the  direction  of  the  main  drainage-lines  or  principal 
rivers,  while  the  subsequent  slopes  due  to  erosion  will 
usually  be  manifested  by  the  course  of  tributary 
streams.  During  the  progress  of  denudation,  how- 
ever, many  modifications  of  the  drainage  will  be 
brought  about.  Cases  of  the  capture  of  principal 
rivers  by  lateral  streams  working  their  way  back  or 
across  the  strike  can  hardly  fail  to  occur,  and  these 
and  other  changes  may  render  the  original  drainage- 
lines  obscure  and  hard  to  trace. 

To  such  an  extent  have  many  ancient  plateaux  of 
erosion    been    denuded,   so    deeply   have    they  been 


i32  EARTH  SCULPTURE 

trenched,  that  their  surface  has  become  resolved  into 
a  truly  mountainous  region,  wherein  all  the  elevations 
are  mountains  of  circumdenudation,  the  tops  of  which 
are  the  only  remaining  relics  of  the  original  plateau- 
surface.  Such  mountains,  owing  generally  to  the 
durability  of  their  rocks  and  the  strength  of  their 
structure,  are  not  so  readily  demolished  as  the  moun- 
tains in  a  range  of  recent  uplift.  They  may  not  often 
emulate  these  in  height  and  grandeur,  their  profiles 
may  as  a  rule  be  less  wild  and  irregular ;  but  such  is 
not  always  the  case.  When  a  plateau  of  erosion 
stands  at  a  great  elevation,  the  mountains  carved  out 
of  it  are  apt  to  rival  the  boldest  and  most  abrupt  of 
Alpine  heights.  Such  abrupt  slopes  and  the  profound 
valleys  that  intervene  are  the  result  of  relatively  rapid 
and  powerful  vertical  erosion.  But  when  a  plateau 
has  only  a  moderate  elevation,  the  configuration  of  its 
mountains  tends  to  be  less  abrupt,  and  to  approximate 
in  character  to  that  attained  by  a  true  mountain-chain 
during  the  period  of  its  maturity,  when  all  weak 
structures  have  been  demolished  and  the  surface  no 
longer  coincides  with  the  folds  of  the  strata.  And 
this  is  just  what  might  have  been  expected,  when  it 
is  borne  in  mind  that  in  each  case  the  fundamental 
geological  structure  is  the  same.  A  mountain-chain 
is  composed  mainly  of  highly  flexed  and  folded  rocks. 
Subjected  to  erosion,  the  whole  region  is  remodelled 
and  eventually  reduced  to  a  base-level.  But  the  rock- 
structure  remains  ;  the  plain  of  erosion  is  composed, 
just  as  the  mountains  were,  of  highly  flexed  and  folded 


LAND-FORMS  TN  HIGHLY  FOLDED  STRATA     133 

rocks.  When  that  plain  is  uplifted  en  masse  to  form 
a  plateau  it  is  obvious  that  epigene  action  must  tend 
to  evolve  out  of  the  plateau  mountains  and  ridges 
which,  in  their  form  and  alignment,  will  closely  re- 
semble those  that  existed  over  the  same  area  before 
the  old  plain  of  erosion  had  come  into  existence.  The 
rocks  and  rock-arrangements,  being  the  same  in  both 
cases,  must  under  denudation  tend  to  produce  a  sim- 
ilar configuration.  No  doubt  there  might  be  certain 
contrasts,  but  these  would  not  be  due  so  much  to 
geological  structure  as  to  changes  in  the  character  of 
the  rocks.  The  planing  away  of  great  mountain- 
masses  might  well  expose  quite  a  different  series  of 
rocks,  and  these,  when  the  region  was  again  uplifted 
and  carved  into  hill  and  valley,  would  doubtless 
weather  differently  from  the  rock-masses  under  which 
they  formerly  lay  buried.  But  the  general  geological 
structure  remaining  the  same,  mountains  and  ridges 
would  necessarily  be  developed  along  the  old  lines. 

We  may  now  consider  the  structure  of  certain  plat- 
eaux of  erosion  which  there  is  every  reason  for  be- 
lieving existed  at  one  time  as  plains — plains  which 
had  previously  replaced  mountain-systems.  A  good 
example  is  ready  to  our  hand  in  the  Southern  Uplands 
of  Scotland — that  belt  of  high  ground  which  is  drained 
by  the  Clyde,  the  Doon,  and  other  streams  flowing 
north-west,  and  by  the  Cree,  the  Dee,  the  Nith,  and 
the  Annan  flowing  south-east.  The  north-east  sec- 
tion of  the  region  is  traversed  by  the  Tweed,  with  an 
easterly  to  north-easterly  course  ;  while  the  extreme 


i34  EARTH  SCULPTURE 

south-west  portion  is  watered  by  the  Stinchar,  flowing 
in  a  south-west  direction.  The  whole  area  drained 
by  those  rivers  and  streams  might  be  described  as  a 
broad  undulating  plateau,  furrowed  and  trenched  by 
narrower  and  wider  valleys.  The  mountains  are  some- 
what tame  and  monotonous — flat-topped  elevations 
with  broad,  rounded  shoulders  and  smooth  grassy 
slopes.  The  rocks  composing  the  region  consist  for 
the  most  part  of  greywackes  and  shales,  the  former 
being  usually  hard  greyish-blue  rocks  arranged  in 
beds  of  variable  thickness.  They  are  much  more 
abundantly  developed  than  the  shales  which  are  asso- 
ciated with  them,  although  now  and  again  the  latter 
attain  considerable  importance.  The  strata  usually 
dip  at  high  angles,  often  approaching  the  vertical,  and, 
the  same  beds  coming  again  and  again  to  the  surface, 
it  is  obvious  that  we  are  dealing  here  with  a  vast  suc- 
cession of  steeply  inclined  and  closely  pressed  anti- 
clinal and  synclinal  folds.  In  many  natural  exposures, 
as  on  the  coast  and  in  the  valleys,  the  intensely  folded 
character  of  the  strata  is  clearly  revealed.  Obviously 
the  strata  have  been  squeezed  together,  and  affected 
in  precisely  the  same  way  as  the  rocks  of  the  Alps. 
Frequently,  indeed,  we  find  that  overthrusting  has 
taken  place,  the  rocks  having  yielded  to  tangential 
pressure  by  shearing.  The  general  trend  or  "  strike  " 
of  the  strata  is  from  south-west  to  north-east,  while 
the  dip  is  sometimes  north-west,  sometimes  south-east, 
changing  now  and  again  very  rapidly,  at  other  times 
remaining  constant  for  long  distances.  In  the  former 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     135 

case  the  folds  are  not  infrequently  approximately 
symmetrical  ;  in  the  latter  they  are  necessarily  un- 
symmetrical.  In  a  word,  the  geological  structure  is 
that  which  characterises  all  mountains  of  elevation 
like  the  Alps.  Nor  can  we  reasonably  doubt  that 
when  the  folding  and  fracturing  took  place  the  crust 
bulged  up  and  a  series  of  superficial  ridges  and  hol- 
lows— a  true  mountain-chain — came  into  existence. 
That  was  a  very  long  time  ago,  however,  for  the  up- 
lift dates  back  towards  the  close  of  Silurian  times. 
Then  followed  a  protracted  period  of  denudation, 
during  which  our  mountains  of  folded  rocks  must 
have  passed  through  the  various  stages  of  adolescence, 
maturity,  and  old  age.  Much  of  the  region  was  re- 
duced to  the  condition  of  a  low  plain,  diversified  in 
part  by  swelling  hills  of  less  and  greater  height.  All 
this  work  had  been  accomplished,  and  the  degraded 
hills  were  continuing  to  crumble  awray,  when  the 
whole  region  was  once  more  uplifted,  and  so  converted 
into  a  table-land  or  plateau  with  an  undulating  surface. 
This  movement  of  elevation  had  been  completed, 
and  renewed  erosion  had  furrowed  and  trenched  the 
plateau  to  some  extent,  before  the  beginning  of  Old 
Red  Sandstone  times,  for  the  lowest  or  bottom  beds 
of  the  Old  Red  Sandstone  series  here  and  there  oc- 
cupy valleys  carved  out  of  the  underlying  Silurian 
greywacke  and  shale.  To  what  extent  the  plateau 
was  submerged  during  the  Old  Red  Sandstone  period 
we  cannot  tell.  Probably  the  submergence  was  great- 
est over  the  north-east  portion  of  the  region,  for  it  is 


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EARTH  SCULPTURE 

in  that  quarter  that  we  meet  with 
the  most  extensive  and  continu- 
ous accumulations  of  Old  Red 
Sandstone  rocks.  Be  that  as  it 
may,  we  know  that  some  time 


before  the  succeeding  Carbon- 
iferous period  re-elevation  en- 
sued and  a  new  cycle  of  erosion 
was  inaugurated,  during  which 
the  Old  Red  Sandstone  rocks 
and  the  underlying  Silurian 
strata  were  more  or  less  pro- 
foundly denuded.  Thereafter 
followed  an  epoch  of  renewed 
subsidence  on  a  more  extensive 
scale,  when  much  of  the  plateau 
was  drowned  in  the  Carbonifer- 
ous sea,  and  marine  sediments 
of  that  age  were  distributed  over 
areas  which  had  probably  never 
been  overflowed  by  the  waters 
of  Old  Red  Sandstone  times. 
Judging  from  the  present  dis- 
tribution of  the  Carboniferous 
strata,  it  seems  likely  that  the 
plateau  was,  as  before,  more 
deeply  submerged  towards  north- 
east and  south-east  than  in  other 
directions.  So  far  as  we  can  tell, 
the  region  has  never  since  been 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     137 

depressed  below  the  sea,  but  in  succeeding  Permian 
and  Triassic  times  long  stretches  of  inland  lakes  or 
seas  penetrated  into  the  heart  of  the  plateau,  occupy- 
ing hollows  which  were  certainly  in  existence  during 
the  preceding  Carboniferous  period. 

Such,  without  going  into  details,  is  a  general  out- 
line of  the  chief  changes  which  have  taken  place  in 
the  Southern  Uplands  of  Scotland.  A  plateau  which 
came  into  existence  towards  the  end  of  the  Silurian 
period  might  well  be  expected  to  show  a  highly  de- 
nuded aspect.  It  is  true  that  during  Old  Red  Sand- 
stone and  Carboniferous  times  it  was  considerably 
depressed,  and  so  escaped  much  erosion,  but  in  the 
intervals  separating  those  stages  denudation  must 
have  been  in  active  progress,  as  it  has  continued  to 
be  since  the  final  disappearance  of  marine  conditions. 
No  doubt  much  rock  has  been  removed  from  the 
whole  surface  of  the  region  in  question.  Not  only 
have  wide  and  deep  valleys  been  excavated,  but  the 
broad-backed  hills  and  mountains  can  hardly  fail  to 
have  been  greatly  reduced  in  height.  It  is  still  pos- 
sible, however,  to  trace  the  general  configuration  of 
the  original  surface.  The  average  slope  of  the  plateau 
appears  to  have  been  towards  the  south-east.  This  is 
indicated  by  the  direction  of  the  principal  rivers — the 
Annan,  the  Nith,  the  Ken,  and  the  Cree.  It  is  fur- 
ther shown  by  the  distribution  of  the  Old  Red  Sand- 
stone and  later  geological  formations.  Thus  strata 
of  Old  Red  Standstone  and  Carboniferous  age  oc- 
cupy the  Merse  and  the  lower  reaches  of  Teviotdale, 


138  EARTH  SCULPTURE 

and  extend  up  the  valleys  of  the  Whiteadder  and  the 
Leader  into  the  heart  of  the  Silurian  uplands.  In 
like  manner  Permian  sandstones  are  well  developed 
in  the  ancient  hollows  of  Annandale  and  Nithsdale. 
Along  the  northern  borders  of  the  Southern  Uplands 
we  meet  with  similar  evidence  to  show  that  even  as 
early  as  the  Old  Red  Sandstone  period  the  ancient 
plateau  along  what  is  now  its  northern  margin  was 
penetrated  by  valleys  that  drained  towards  the  north. 
But  the  main  water-parting  then,  as  now,  lay  not  far 
south  of  this  northern  margin  * ;  in  other  words,  the 
surface  of  the  ancient  plateau,  a  few  miles  back  from 
its  northern  boundary,  sloped  persistently  towards 
the  south-east.  Now  the  strike  or  general  trend  of 
the  strata  throughout  the  whole  of  these  Uplands  is 
south-west  and  north-east.  We  cannot  doubt,  there- 
fore, that  when  the  ancient  plain  of  erosion  was  up- 
lifted, and  so  became  a  plateau,  the  surface  would  be 
marked  by  many  more  or  less  well-defined  ridges  and 
hollows,  probably  none  very  prominent,  but  all  hav- 
ing a  north-east  and  south-west  trend.  The  average 
slope  of  the  surface  being  towards  south-east,  the 

1  Many  modifications  of  the  drainage  have  been  effected  which  cannot  be  re- 
ferred to  here.  It  may  be  pointed  out,  however,  that  the  head-waters  of  the 
Nith  flow  towards  the  north  until  they  reach  the  broad  Nithsdale,  whence  the 
drainage  is  directed  south-east,  so  that  Nithsdale  may  be  said  to  cut  right  across 
the  Uplands  from  north-west  to  south-east.  This  is  probably  a  case  of  capture, 
the  Nith,  working  back,  having  gradually  invaded  the  northern  drainage-area 
and  captured  such  streams  as  the  Afton  and  the  Connel.  The  Clyde  and  the 
Doon  are  the  only  rivers  of  any  size  which  have  preserved  their  north-westerly 
course,  and  the  head-waters  of  the  former  have  just  escaped  capture  by  the 
Tweed. 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     139 

flow  of  the  principal  rivers  would  follow  that  direction, 
they  would  cut  their  channels  across  the  outcrops  of 
the  strata.  But  the  "  corduroy  "character  of  the 
plateau  would  now  and  again  lead  to  occasional  de- 
flections, while  some  streams  and  rivers  would  be 
conducted  for  long  distances  parallel  to  the  strike  of 
the  strata.  In  a  word,  two  sets  of  principal  valleys 
would  tend  to  be  formed,  namely,  transverse  and  longi- 
tudinal valleys.  Examples  of  the  former  have  already 
been  cited,  such  as  the  Cree,  the  Ken,  and  the  Nith, 
and  amongst  the  better-known  longitudinal  valleys 
may  be  mentioned  those  of  the  Teviot,  the  Ettrick, 
and  the  Yarrow.  But  a  glance  at  any  good  map  of 
the  region  will  show  that  all  the  more  important 
streams  have  a  tendency  to  flow  either  in  a  transverse 
or  a  longitudinal  direction,  while  many  run  now  in  one 
of  these  directions  and  now  in  the  other. 

The  Southern  Uplands  thus  prove  to  be  merely  a 
highly  eroded  plateau.  Their  geological  structure 
shows  that  towards  the  close  of  Silurian  times  the 
greywackes  and  shales  were  buckled  up,  folded,  and 
faulted,  and  doubtless  appeared  at  first  as  a  range  of 
true  mountains  of  elevation.  Thereafter  followed  a 
prolonged  period  of  erosion,  interrupted,  it  is  true,  at 
successive  stages  by  partial  submergence,  but  result- 
ing finally  in  the  demolition  of  the  old  mountains  of 
elevation  and  the  conversion  of  the  tract  into  a  plain 
of  erosion.  Then  came  a  final  regional  uplift,  when 
that  plain  was  converted  into  a  plateau,  which  still 
exists,  but  in  a  highly  denuded  and  eroded  condition. 


140  EARTH  SCULPTURE 

The  Northern  Highlands  of  Scotland  might  be 
cited  as  another  plateau  of  erosion  with  a  somewhat 
similar  geological  history.  There,  as  in  the  south, 
there  is  evidence  to  show  that  vast  earth-movements 
resulted,  towards  the  close  of  Silurian  times,  in  the 
formation  of  great  mountains  of  elevation.  The 
thrust-planes  visible  in  the  north-west  part  of  that 
region  are  on  a  much  more  extensive  scale  than  those 
met  with  in  the  Southern  Uplands.  Probably  the 
mountains  of  elevation  which  appeared  over  the  site 
of  the  present  Highlands  were  loftier  and  bolder  than 
the  pre-Devonian  heights  of  Southern  Scotland. 
They  may  quite  possibly  have  rivalled  the  Alps  in 
grandeur,  for  the  folding  and  general  disturbance  of 
the  rocks  are  quite  as  remarkable  as  the  confusion 
seen  in  the  mountains  of  Switzerland.  We  may  well 
believe  that  when  the  Highland  mountains  first  up- 
rose, their  external  form  and  internal  structure  would 
more  or  less  closely  coincide.  No  sooner  had  they 
come  into  existence,  however,  than  the  usual  cycle  of 
erosion  would  commence,  and  it  is  certain  that  after 
a  prolonged  interval  they  were  to  a  large  extent  re- 
duced to  their  base-level — much  of  the  formerly  ele- 
vated area  acquiring  the  character  of  a  plain  of  erosion. 
Subsidence  next  ensued,  and  that  plain  became  grad- 
ually overspread  with  sediment,  several  thousand  feet 
of  Old  Red  Sandstone  strata  being  deposited  on  the 
planed  and  abraded  surface  of  the  ancient  rocks.  At 
a  subsequent  date  the  whole  region  was  uplifted  and 
converted  into  dry  land,  forming  a  plateau  country, 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     141 

which,  so  far  as  we  know,  has  never  since  been  com- 
pletely submerged,  although  it  may  well  have  ex- 
perienced many  oscillations  of  level. 

It  is  out  of  that  ancient  plateau  that  the  Highland 
mountains  have  been  carved.  The  original  surface- 
slope  is,  as  usual  in  such  cases,  indicated  partly  by  the 
direction  of  the  principal  drainage-lines  and  partly  by 
the  summits  of  the  mountains,  which  decline  in  eleva- 
tion as  they  are  followed  outwards  in  the  direction  of 
the  chief  lines  of  drainage.  Again,  the  main  water- 
partings  separating  the  more  extensive  drainage-areas 
of  the  country  mark  out  in  like  manner  the  dominant 
portions  of  the  same  old  plateau-land.  The  water- 
parting  of  the  North-west  Highlands  runs  nearly 
north  and  south,  keeping  quite  close  to  the  western 
shore,  so  that  nearly  all  the  drainage  of  that  region 
flows  inland.  The  average  inclination  of  that  section 
of  the  Highlands  is  therefore  easterly,  towards  Glen- 
more  and  the  Moray  Firth.  In  the  region  east  of 
Glenmore  the  land  slopes  in  the  directions  followed 
by  the  rivers  Spey,  Dee,  and  Tay.  These  two  regions 
—the  North-west  and  the  South-east  Highlands- 
are  separated  by  the  remarkable  depression  of  Glen- 
more,  running  through  Lochs  Linnhe,  Lochy,  and 
Ness,  and  the  further  extension  of  which  towards 
north-east  is  indicated  by  the  straight  coast-line  of  the 
Moray  Firth  as  far  as  Tarbat  Ness.  This  long  de- 
pression marks  a  line  of  fracture  and  displacement  of 
very  great  geological  antiquity.  The  old  plateau  of 
the  Highland  area  was  fissured  and  split  in  two,  that 


142  EARTH  SCULPTURE 

portion  which  lay  to  the  north-west  sinking  along  the 
line  of  fissure  to  a  great  but  unascertained  depth.1 
Thus  the  waters  that  flowed  down  the  slopes  of  the 
north-west  portion  of  the  fractured  plateau  were 
dammed  by  the  long  wall  of  rock  that  rose  upon  the 
south-east  side  of  the  fissure,  and  compelled  to  flow 
off  to  north-east  and  south-west  along  the  line  of  dis- 
placement. The  erosion  thus  induced  sufficed  in 
course  of  time  to  hollow  out  Glenmore  and  all  the 
mountain-valleys  that  open  upon  it  from  the  west. 

The  dominant  portion  of  the  ancient  plateau  east 
of  the  great  fault  is  approximately  indicated  by  a  line 
drawn  from  Ben  Nevis  through  the  Cairngorm  and 
Ben  Muich  Dhui  Mountains  to  Kinnaird  Point. 
North  of  that  line  the  drainage  is  towards  the  Moray 
Firth ;  east  of  it  the  rivers  discharge  to  the  North 
Sea ;  while  an  irregular  winding  line,  drawn  from  Ben 
Nevis  eastward  through  the  Moor  of  Rannoch,  and 
southward  to  Ben  Lomond,  forms  the  water-parting 
between  the  North  Sea  and  the  Atlantic,  and  probably 
marks  approximately  another  dominant  area  of  the 
fractured  table-land. 

The  geological  structure  of  the  Highlands  agrees 
so  far  with  that  of  the  Southern  Uplands,  that  the 
dominant  "strike"  of  the  strata  is  south-west  and 
north-east.  This,  therefore,  is  the  trend  of  the  flexures 
and  folds  and  of  all  the  larger  normal  faults  and  great 

1  It  is  probable  that  movements  have  taken  place  again  and  again  at  different 
periods  along  this  line  of  weakness,  and  these  movements  may  not  always  have 
been  in  one  direction. 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     143 

thrust-planes.  Now  such  a  structure  would  naturally 
determine  the  disposition  of  the  surface-features 
worked  out  by  erosion.  Before  the  beginning  of  the 
Old  Red  Sandstone  period,  the  pre-existing  mount- 
ains of  uplift  had  been  largely  degraded  to  a  base- 
level.  Much  of  the  region,  in  other  words,  had  been 
converted  into  a  plain  of  erosion,  which  subsequently 
became  depressed  and  buried  under  thick  accumula- 
tions of  sediment,  derived  in  chief  part  from  the  de- 
nudation of  such  parts  of  the  Highland  area  as  still 
remained  in  the  condition  of  dry  land.  After  the 
deposition  of  the  Old  Red  Sandstone  the  whole  region 
was  elevated  en  masse,  and  converted  into  a  plateau 
or  table-land.  The  surface  of  that  plateau  would 
doubtless  be  somewhat  undulating  and  diversified. 
Probably  the  "  stumps  "  of  the  highly  denuded  mount- 
ains, which  had  supplied  materials  for  the  formation 
of  the  Old  Red  Sandstone,  still  formed  dominant 
areas.  But  wide  regions  had  been  planed  down,  and 
these  would  be  marked  by  a  kind  of  "  corduroy " 
structure — parallel  lines  of  escarpment  and  ridges 
with  intervening  hollows,  corresponding  to  the  suc- 
cessive outcrops  of  "  harder "  and  "softer"  rocks. 
The  regions  overspread  by  the  Old  Red  Sandstone, 
on  the  other  hand,  would  be  approximately  level, 
sloping  gently,  however,  towards  the  north,  north- 
east, and  south-east.  We  may,  therefore,  conceive 
the  surface  of  the  ancient  Highland  Plateau  to  have 
been  from  the  first  more  irregular  than  that  of  the 
Southern  Table-land.  The  primeval  rivers  would 


144  EARTH  SCULPTURE 


doubtless  follow  the  average  slopes  of  the  plateau, 
and  would  thus  sometimes  cross  the  outcrops  at  all 
angles,  and  sometimes  flow  in  the  direction  of  the 
strike  for  longer  or  shorter  distances.  The  great  de- 
pression on  the  line  of  the  Caledonian  Canal,  although 
partially  rilled  with  the  sediments  of  Old  Red  Sand- 
stone times,  probably  still  formed  a  well-marked 
feature  at  the  surface  of  the  plateau  when  this  was 
first  uplifted.  And  the  same  may  well  have  been  the 
case  with  many  other  lines  of  fracture.  In  short, 
although  the  average  slope  of  the  ground  determined 
the  general  direction  of  the  drainage,  the  corrugated 
and  often  much  diversified  surface  of  the  plateau 
must  have  led  to  endless  deflection  of  the  water-flow. 
Again,  as  erosion  proceeded,  and  the  valleys  were  cut 
deeper  and  deeper,  many  modifications  of  the  drainage 
would  naturally  arise,  cases  of  the  "  capture"  of  one 
stream  by  another  having  been  of  common  occurrence. 
It  is  not,  however,  with  the  history  of  such  changes 
that  we  have  to  do,  but  rather  with  the  character  of 
the  existing  valleys  and  mountains  which  have  been 
carved  and  chiselled  out  of  the  ancient  plateau.  Of 
the  valleys  it  may  be  said  in  general  terms  that  they 
are  all  valleys  of  erosion.  Many  have  been  hollowed 
out  along  the  outcrops,  and  are  thus  longitudinal,  while 
others  have  been  cut  out  across  the  "  strike,"  and  to 
this  extent  are  transverse.  Some  of  the  former  are 
of  primeval  antiquity :  they  correspond  in  direction 
not  only  with  the  strike  of  the  strata,  but  with  what 
seems  to  have  been  the  original  slope  of  the  plateau, 


LAND-FORMS  IN  HIGHLY  FOLDED  STRA  TA     145 

the  valley  of  the  Spey  being  the  most  conspicuous  ex- 
ample. The  transverse  valleys,  represented  typically 
by  Glen  Garry  and  the  valley  of  the  Tay,  are  obvi- 
ously also  of  great  age,  since  they  in  like  manner  in- 
dicate the  general  slope  of  the  plateau  in  the  regions 
where  they  occur.  A  large  proportion  of  the  longi- 
tudinal valleys  that  drain  into  these  transverse  valleys 
are  in  all  probability  of  subsequent  origin,  although 
some  of  them  may  have  been  outlined  at  as  early  a 
date  as  the  latter.  Although  none  of  the  longitudinal 
valleys  can  be  described  as  synclinal,  they  may  all 
nevertheless  be  termed  structural,  inasmuch  as  they 
coincide  with  the  strike  of  the  rocks.  So  likewise  we 
may  term  Glenmore  a  structural  hollow,  since  it  occurs 
along  a  line  of  fracture  ;  and  the  same  is  the  case  with 
Glen  Docherty  and  Loch  Maree.  These  lines  of 
fractures  no  doubt  showed  at  the  surface  of  the  plateau 
when  it  wa's  first  uplifted,  and  so  determined  the  di- 
rection of  drainage  and  erosion.  But  all  the  valleys 
as  we  now  see  them  are  valleys  of  erosion,  their  di- 
rection having  been  determined  sometimes  by  the 
average  slope  of  the  plateau,  sometimes  by  the  geo- 
logical structure. 

The  mountains  of  the  Highlands  are  likewise  monu- 
ments of  erosion,  owing  their  existence  as  such  some- 
times to  the  relative  durability  of  their  materials, 
sometimes  to  their  geological  structure,  or  to  both 
causes  combined.  They  are  all,  without  exception, 
subsequent  or  relict  mountains.  Thus,  in  the  follow- 
ing section  from  Glen  Lyon  to  Carn  Chois  we  see 


146  EARTH  SCULPTURE 

that  the  present  configuration  of  the  surface  does  not 
coincide  with  the  complicated  underground  structure. 
It  is  the  same,  indeed,  throughout  all  the  Highland 
area.  Take  a  section  across  any  portion  of  that 
region,  and  you  shall  find  that  the  more  continuous 
"  ranges  "  are  developed  along  the  outcrops  —  they  are, 
in  short,  escarpment  mountains.  So  great  has  been 
the  erosion,  however,  within  such  "  ranges,"  that  their 
alignment  usually  becomes  obscured,  and  we  are  con- 
fronted by  confused  groups  of  mountains,  drained  by 
streams  flowing  in  every  possible  direction.  "  Any 


Ck«ii 


FIG.  59.      SECTION  FROM  GLEN  LYON  TO  CARN  CHOIS.    (Geol.  Survey.} 

mt  mica-schist,  etc.  ;  A  limestone ;  gr,  greywacke,  etc.  ;  j,  amphibolite  schist ;  g,  granite  ;  <£, 

diorite ;  /,  fault. 

wide  tract  of  the  Highlands,"  as  we  have  elsewhere 
remarked,  "when  viewed  from  a  commanding  posi- 
tion, looks  like  a  tumbled  ocean,  in  which  the  waves 
appear  to  be  moving  in  all  directions.  One  is  also 
impressed  with  the  fact  that  the  undulations  of  the 
surface,  however  interrupted  they  may  be,  are  broad  ; 
the  mountains,  however  much  they  may  vary  in  their 
configuration  according  to  the  character  of  the  rocks, 
are  massive  and  generally  round-shouldered,  and  often 
somewhat  flat-topped  ;  while  there  is  no  great  dis- 
parity of  height  amongst  the  dominant  points  of  any 
individual  group.  Let  us  take,  for  example,  the  knot 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     147 

of  mountains  between  Loch  Maree  and  Loch  Tor- 
ridon.  There  we  have  a  cluster  of  eight  mountain- 
masses,  the  summits  of  which  do  not  differ  much  in 
elevation.  Thus  in  Llathach  two  points  reach  3358 
feet  and  3486  feet ;  in  Beinn  Alligin  there  are  also 
two  points  reaching  3021  feet  and  3232  feet  respect- 
ively ;  in  Beinn  Dearg  we  have  a  height  of  2995  feet ; 
in  Beinn  Eighe  are  three  dominant  points,  3188  feet, 
3217  feet,  and  3309  feet.  The  four  masses  to  the 
north  are  somewhat  lower,  their  elevations  being  2860 
feet,  2370  feet,  and  2892  feet.  The  mountains  of 
Lochaber  and  the  Monadhliath  Mountains  exhibit 
similar  relationships  ;  and  the  same  holds  good 
with  all  the  mountain-groups  of  the  Highlands.  One 
cannot  doubt  that  such  relationship  is  the  result 
of  denudation.  The  mountains  are  monuments  of 
erosion  ;  they  are  the  wreck  of  an  old  table-land, 
the  upper  surface  and  original  height  of  which  are 
approximately  indicated  by  the  summits  of  the  vari- 
ous mountain-masses  and  the  direction  of  the  princi- 
pal rivers.  If  we  in  imagination  fill  up  the  valleys 
with  the  rock-material  which  formerly  occupied  their 
place,  we  shall  in  some  measure  restore  the  general 
aspect  of  the  Highland  area  before  its  mount- 
ains began  to  be  shaped  out  by  Nature's  saws  and 
chisels." 

A  table-land  of  erosion,  long  exposed  to  denuda- 
tion, must  obviously  pass  through  the  same  phases 
as  a  plateau  of  accumulation.  The  elevated  plain  of 
complicated  geological  structure  is  first  traversed  by 


1 48  EARTH   SCULPTURE 

rivers,  the  courses  of  which  are  determined  by  the 
average  slope  of  the  land.  As  valleys  are  deepened 
and  widened,  and  the  whole  surface  comes  under  the 
influence  of  the  epigene  agents,  new  tributary  streams 
continue  from  time  to  time  to  make  their  appearance, 
and  eventually  a  perfect  network  of  drainage-lines  is 
established.  Wherever  the  rocks  yield  most  readily 
to  erosion  hollows  are  formed,  and  many  of  these 
will  necessarily  coincide  with  the  outcrop  or  strike 
of  the  strata.  Longitudinal  valleys  thus  tend  to  be 
developed.  As  denudation  proceeds,  the  capture 
of  streams  by  rivers  and  of  rivers  by  streams  often 
takes  place,  and  the  hydrographic  system  becomes 
more  or  less  modified,  but  the  general  direction 
of  the  chief  lines  of  drainage  remains  unchanged. 
Eventually  transverse  rivers  are  found  cutting  across 
mountain-ridge  after  mountain-ridge,  the  latter  hav- 
ing only  been  developed  after  the  rivers  had  come* 
into  existence.  With  the  deepening  and  widening 
of  the  main  valleys,  and  the  continual  multiplica- 
tion of  subsidiary  hollows  by  springs,  torrents,  and 
streams,  the  whole  plateau  eventually  becomes  cut 
up  into  irregular  segments  of  every  shape,  form,  and 
size — a  rolling  mountain-land.  Waterfalls,  rapids, 
and  other  irregularities  have  now  disappeared  from 
the  courses  of  the  older  rivers  and  streams,  except, 
it  may  be,  towards  their  heads,  where  more  or  less 
numerous  feeders  are  busy  cutting  their  way  back 
into  the  mountains.  Should  the  base-level  be  main- 
tained, the  process  of  denudation  must  continue  until 


LAND-FORMS  IN  HIGHLY  FOLDED  STRATA     149 

the  rolling  mountain-land  is  finally  reduced  and  re- 
solved once  more  into  a  plain  of  erosion. 

It  is  seldom,  however,  that  a  cycle  of  erosion  is 
allowed  to  pass  through  all  its  stages.  The  study  of 
many  ancient  plateaux  has  shown  that  the  base-level 
is  not  infrequently  disturbed — sometimes  by  eleva- 
tion, at  other  times  by  depression.  Long  before  the 
eroded  plateau  has  been  completely  reduced,  subsid- 
ence may  ensue,  and  the  drowned  land  may  then 
become  buried  under  vast  accumulations  of  marine 
sediments.  Should  the  region  be  once  more  up- 
heaved and  converted  into  dry  land,  streams  and 
rivers  will  again  come  into  existence,  and  flow  in 
directions  determined  by  the  slopes  of  the  surface. 
Thus  ere  long  another  hydrographic  system  will  be 
developed  which  may  differ  entirely  from  its  prede- 
cessor, both  as  regards  direction  and  arrangement. 
As  the  rivers  cut  their  way  down  through  the  super- 
imposed marine  strata  they  will  eventually  reach  the 
buried  land-surface,  across  which  they  will  run  with- 
out any  reference  to  the  former  configuration.  Should 
the  base-level  remain  unchanged,  a  time  will  come 
when  the  overlying  marine  strata  will  be  entirely 
removed,  but  the  direction  and  general  arrangement 
of  the  river-system  acquired  when  the  land  was  new- 
born will  be  maintained.  Thus  the  direction  of  many 
transverse  rivers,  which  in  ancient  plateau-lands  are 
found  cutting  across  mountains  of  every  shape  and 
disposition,  have  not  infrequently  been  determined 
by  the  surface-slope  of  overlying  masses,  almost  every 
vestige  of  which  has  since  disappeared. 


CHAPTER   VII 

LAND-FORMS  IN  REGIONS  AFFECTED  BY 
NORMAL  FAULTS  OR  VERTICAL  DISPLACEMENTS 

NORMAL    FAULTS,     GENERAL    FEATURES    OF THEIR    CONNECTION 

WITH  FOLDS THEIR   ORIGIN HOW     THEY     AFFECT    THE    SUR- 
FACE  FAULTS  OF  THE  COLORADO  REGION,  AND  OF   THE  GREAT 

BASIN DEPRESSION    OF   THE    DEAD    SEA     AND    THE    JORDAN 

LAKE-DEPRESSIONS      OF      EAST      AFRICA FAULTS     OF     BRITISH 

COAL-FIELDS — BOUNDING     FAULTS    OF     SCOTTISH     HIGHLANDS 

AND     LOWLANDS FAULT-BOUNDED     MOUNTAINS GENERAL 

CONCLUSIONS. 

IN  Chapter  III.  a  short  account  was  given  of 
the  dislocations  or  fractures  by  which  rocks  are 
frequently  traversed.  These,  as  we  saw,  are  of  two 
kinds — normal  faults  and  reversed  faults  or  over- 
thrusts.  The  latter  have  been  sufficiently  referred 
to  in  connection  with  the  appearances  presented  by 
highly  flexured  strata,  amongst  which,  indeed,  they 
are  most  usually  encountered.  Normal  faults  of  vari- 
ous importance  may  likewise  often  be  seen  travers- 
ing areas  of  disturbed  and  contorted  rocks.  When 
such  is  the  case,  however,  the  larger  of  these  faults 
not  infrequently  prove  to  be  of  later  date  than  the 
flexures  and  thrust-planes.  The  latter  are  the  result 

150 


VERTICAL   DISPLACEMENTS  151 

of  former  horizontal  movements  of  the  crust  ;  the 
normal  faults,  on  the  other  hand,  are  vertical  dis- 
placements due  to  later  movements  of  direct  subsid- 
ence. It  will  be  understood,  therefore,  that  reversed 
faults  or  overthrusts  are  practically  confined  to  regions 
of  highly  flexed  and  contorted  strata,  while  normal 
faults  traverse  every  kind  of  geological  structure.  The 
latter,  however,  are  certainly  best  displayed  in  areas 
of  horizontal  and  moderately  inclined  strata,  while 
they  often  form  lines  of  separation  between  these  and 
contiguous  areas  of  highly  disturbed  rock-masses. 

The  amount  of  downthrow  of  normal  faults  is  very 
variable.  Sometimes  it  does  not  exceed  a  few  feet 
or  yards,  in  other  cases  it  may  reach  thousands  of 
feet,  so  that  strata  of  vastly  different  ages  may  be 
brought  into  juxtaposition.  The  smaller  faults  usu- 
ally extend  for  very  short  distances,  while  the  larger 
ones  may  continue  for  hundreds  or  even  thousands 
of  miles.  The  course  of  great  faults  is  usually 
approximately  straight,  but  not  infrequently  it  is 
curved.  Very  often  they  are  accompanied  by  a  series 
of  smaller  parallel  dislocations ;  and  now  and  again, 
in  place  of  one  great  fault,  with  accompanying  minor 
dislocations,  we  may  find  a  series  of  more  or  less 
closely  set  parallel  minor  faults.  When  the  down- 
throw of  all  these  minor  faults  is  in  one  and  the  same 
direction,  the  result  is  practically  the  same  as  if  there 
had  been  only  one  major  dislocation  with  a  large 
downthrow.  Another  fact  may  be  noted :  faults, 
especially  large  ones,  often  split  up,  as  it  were,  into 


152  EARTH  SCULPTURE 

two  or  more.  A  major  fault  may  begin  as  a  mere 
crack  or  fracture,  with  little  or  no  accompanying 
rock-displacement.  But  as  it  continues  the  amount 
of  downthrow  gradually  increases  until  a  maximum  is 
reached,  after  which  the  displacement  usually  de- 
creases until  finally  the  fault  dies  out.  In  not  a  few 
cases,  however,  the  degree  of  downthrow  varies  very 
irregularly. 

Frequently  faults  are  intimately  connected  with 
folds  and  flexures.  This  is  shown  at  once  by  the 
fact  that  large  dislocations  very  often  trend  in  the 
same  direction  as  the  strike  of  the  strata.  Now  and 
again,  indeed,  when  a  large  fault  can  be  followed  to 
the  end,  it  is  found  gradually  to  die  out  in  a  fold  or 
flexure.  In  other  words,  what  is  a  fault  in  one  place 
is  represented  elsewhere  by  a  flexure.  It  is  not  hard 
to  see  how  that  should  be.  Strain  or  tension  must 
obviously  be  set  up  along  the  margin  of  a  sinking 
area.  If,  for  example,  subsidence  should  take  place 
within  an  area  of  horizontal  strata,  the  horizontal 
position  of  the  rocks  along  the  margin  of  the  sinking 
area  will  be  interfered  with.  The  pull  or  drag  of  the 
descending  mass  will  cause  the  strata  of  the  adjacent 
relatively  stable  area  either  to  bend  over  or  snap 
across.  Should  the  movement  be  slow  and  pro- 
tracted, the  rocks  will  probably  at  first  yield  by 
bending ;  but  as  the  movement  continues  they  will 
eventually  give  way,  and  a  fold  will  thus  be  replaced 
by  a  fracture.  Towards  either  end  of  such  a  fault, 
therefore,  we  should  expect  it  to  die  out  into  a  simple 


VERTICAL  DISPLACEMENTS  153 

flexure  or  monoclinal  fold.  Probably  most  normal 
faults  are  in  this  way  preceded  by  folding,  except  in 
cases  where  they  have  been  more  or  less  suddenly 
produced. 

Although  normal  faults  may  be  looked  upon  as  the 
result  of  direct  subsidence,  it  is  obvious  that  in  some 
cases  they  may  well  have  resulted  from  movements 
of  elevation.  During  the  slow  uplifting  of  a  broad 
plateau  strain  and  tension  will  come  into  play  along 
the  margin  of  the  rising  area.  Folds  will  thus  be 
formed,  and  these  will  be  replaced  eventually  by  frac- 
tures and  displacements.  The  resulting  structure 


I 

FIG.   60.    SECTION  OF  NORMAL  FAULT. 

will  thus  be  practically  the  same  as  if  the  folding  and 
faulting  had  been  produced  by  a  movement  of  subsid- 
ence. Thus  in  Fig.  60  the  fault  /  might  have  been 
caused  either  by  the  direct  subsidence  of  the  strata 
at  x  or  by  the  elevation  of  the  strata  at  a. 

There  is  reason  to  believe  that  some  large  faults 
have  resulted  from  crustal  movements  continued 
through  long  periods  of  time.  The  rock-displace- 
ments may  have  been  very  slowly  and  gradually  ef- 
fected, or  the  movement  may  have  been  more  rapid, 
but  interrupted  again  and  again  by  longer  or  shorter 

tuses.      Or,  again,  the  rate  of  movement  may  have 


154  EARTH  SCULPTURE 

varied  from  time  to  time,  and  occasionally  it  may  even 
have  been  sudden  and  catastrophic.  But  such  evi- 
dence as  we  have  would  lead  us  to  infer  that  vertical 
displacements,  whether  the  result  of  downward  or  of 
upward  movements,  have  not  been  more  rapidly  ef- 
fected than  horizontal  deformations.  No  doubt  a 
sudden  dislocation  of  the  crust  of  large  extent  would 
show  directly  at  the  surface.  But  somewhat  similar 
results  would  follow  if  the  dislocation,  without  being 
quite  sudden,  were  yet  to  be  developed  more  rapidly 
than  the  rate  of  superficial  erosion  and  denudation. 
Cases  of  the  kind  are  well  known,  and  to  some  of 
these  reference  will  presently  be  made.  It  is  with 
faulted  rocks,  however,  as  with  folded  mountains  : 
when  movement  has  ceased  the  Inequalities  caused  at 
the  earth's  surface  tend  to  be  reduced  and  greatly 
modified.  The  epigene  forces  are  untiring  in  their 
action,  so  that  in  course  of  time  areas  of  direct  sub- 
sidence tend  to  become  filled  up  and  the  surrounding 
high-lying  tracts  to  be  worn  down.  To  such  an  ex- 
tent has  this  taken  place,  that  in  the  case  of  certain 
great  faults  of  high  geological  antiquity  no  inequality 
at  the  surface  indicates  their  presence,  and  it  is  only 
by  studying  the  geological  structure  that  we  are  able 
to  ascertain  that  such  dislocations  exist. 

Bearing  in  mind  the  activity  of  the  denuding  agents, 
we  might  expect  that  normal  faults  of  geologically  re- 
cent date  should  show  most  prominently  at  the  surface. 
And  this  to  a  large  extent  is  doubtless  true.  Never- 
theless, as  we  shall  learn  by-and-by,  there  are  certain 


VERTICAL  DISPLACEMENTS  155 

faults  of  prodigious  antiquity  which  still  cause  very 
marked  inequalities  at  the  surface.  These  often  form 
the  boundaries  between  highlands  and  lowlands.  In 
such  cases,  however,  the  disparity  of  level  is  due  not 
so  much  to  vertical  displacement,  as  to  the  fact  that 
the  lowlands  are  usually  composed  of  less  enduring 
materials  than  those  which  enter  into  the  framework 
of  the  adjacent  highlands.  When  a  fault  of  great 
age  traverses  strata  of  much .  the  same  consistency 
(say  sandstones  and  shales),  the  rocks  on  either  side 
of  the  dislocation,  we  find,  have  been  planed  down  to 


FIG.  61.    NORMAL  FAULT,  WITH  HIGH  GROUND  ON  DOWNTHROW  SIDE. 

the  same  level.  Thus  in  the  low-lying  coal-fields  of 
Scotland  the  gently  undulating  surface  gives  no  in- 
dication of  the  presence  of  the  numerous  dislocations 
which  have  been  detected  underground.  Downthrows 
of  hundreds  of  feet  give  rise  to  no  superficial  inequal- 
ities. It  is  only  when  one  of  these  faults  has  brought 
relatively  hard  and  soft  rocks  into  juxtaposition  that 
a  marked  surface-feature  results.  And  in  this  case 
the  hard  rock  invariably  rises  above  the  level  of  the 
soft  rock,  no  matter  on  which  side  of  the  dislocation 
it  happens  to  lie.  Thus  in  Fig.  61  the  hard  rock  a 
forms  an  eminence,  although  it  is  on  the  downthrow 
side  of  the  fault,  simply  because  it  has  withstood  denud- 


156  EARTH  SCULPTURE 

ation  more  effectually  than  the  soft  rock  (6).  In  Fig.  62, 
again,  it  is  obvious  that  the  high  ground  at  x  owes  its 
origin  to  the  presence  of  the  relatively  hard  rock  (/£). 
To  this  matter,  however,  we  shall  return  in  the  sequel. 
Meanwhile  we  must  consider,  first,  the  appearances 
presented  in  regions  where  vertical  movements  of  the 
crust  have  taken  place  within  relatively  recent  times/ 
The  Colorado  Plateau  affords  some  excellent 
examples  of  simple  folds  and  normal  faults  of  com- 
paratively recent  age.  These  have  often  profoundly 
affected  the  surface,  lines  of  cliffs  and  bold  escarp- 
ments rising  along  the  high  side  of  each  dislocation. 


FIG.  62.     NORMAL  FAULT,  WITH  HIGH  GROUND  ON  UPCAST  SIDE. 

The  plateau,  in  short,  has  been  split  across  by  well- 
marked  normal  faults,  some  of  which  can  be  followed 
for  hundreds  of  miles.  Yet  the  strata  on  both  sides 
of  such  dislocations  are  of  much  the  same  character 
and  consistency.  Here,  then,  it  might  be  supposed 
that  the  fracturing  and  displacement  had  been  sud- 
denly effected.  There  is  striking  evidence,  however, 
to  show  that  such  has  not  been  the  case.  Although 
some  of  the  faults  referred  to  have  a  downthrow  of 
several  thousand  feet,  yet  they  have  had  no  effect  in 
disturbing  the  course  of  the  Colorado  River,  which  tra- 
verses the  faulted  region.  The  same,  as  we  have  seen, 


VERTICAL  DISPLACEMENTS 


157 


holds  true  with  regard  to  the  flexures  of  that  area. 
It  is  obvious,  in  a  word,  that  the  process  of  flexuring 
and  faulting  has  proceeded  so  slowly  that  the  river 
has  been  able  to  saw  its  way  across  the  inequalities 
as  fast  as  these  appeared.  But  while  the  rate  of 
river  erosion  has  equalled  that  of  crustal  movement, 
the  denudation  of  the  plateau  outside  of  the  river- 
courses  has  not.  Deformation  and  dislocation  of  the 
plateau  have  thus  given  rise  to  marked  surface-feat- 
ures. Yet  even  in  the  case  of  these  relatively  young 
faults  we  find  that  the  features  determined  by  them 
have  been  very  considerably  modified  by  denudation. 
In  the  following  section,  for  example,  we  see  three 


FIG.  63.     FAULTS  IN  QUEANTOWEEP  VALLEY,  GRAND  CANON  DISTRICT. 

(Button. ) 

faults  of  1 300  feet,  300  feet,  and  800  feet  displacement 
respectively  traversing  the  same  series  of  strata,  and 
yet  giving  rise  to  marked  inequalities  at  the  surface. 
The  dotted  lines,  however,  show  to  what  an  extent 
these  features  have  been  modified  by  denudation. 
There  is  an  obvious  tendency  of  the  escarpments  and 
cliffs  to  become  benched  back  as  they  retreat,  so  that 
they  do  not  show  the  abrupt  character  which  they 
would  have  possessed  had  no  superficial  waste  accom- 


158  EARTH  SCULPTURE 

panied  and  succeeded  the  crustal  movements.     (See 
Fig.  63.) 

In  the  Great  Basin  that  extends  between  the  bold 
escarpment  of  the  Sierra  Nevada,  on  the  one  hand,  and 
the  Wahsatch  Mountains  on  the  other,  we  encounter 
another  series  of  large  faults,  which  have  deter- 
mined the  leading  features  of  the  region.  It  would 
appear  that  the  area  of  the  Great  Basin  formerly 
attained  a  considerably  greater  elevation  than  at  pre- 
sent. Towards  the  close  of  Tertiary  times  the  whole 
of  this  area,  including  the  adjacent  Sierra  Nevada 
and  the  Wahsatch  Mountains,  was  upheaved  in  the 
form  of  a  broad  arch.  The  crust  thus  subject  to  ten- 
sion yielded  by  cracking  across,  and  a  system  of  long 
parallel  north  and  south  fissures  was  formed.  In 
other  words,  the  broad  arch  was  split  into  a  series  of 
oblong  blocks  many  miles  in  extent.  When  the 
movement  of  elevation  ceased  and  subsidence  en- 
sued, the  shattered  crust  settled  down  unequally 
between  the  Sierra  Nevada  in  the  west  and  the  Wah- 
satch Mountains  in  the  east.  The  amount  of  dis- 
placement along  the  margins  of  the  Great  Basin  is 
very  great ;  the  fault  at  the  base  of  the  Sierra,  for 
example,  is  estimated  to  be  not  less  than  15,000  feet, 
while  that  which  severs  the  Basin  from  the  Wahsatch 
Mountains  is  also  very  great.  The  numerous  parallel 
ranges  that  diversify  the  surface  of  the  Great  Basin 
itself  are  simply  oblong  crust-blocks,  brought  into 
position  by  normal  faults.  Being  of  so  recent  an  age, 
they  have  suffered  comparatively  little  modification. 


VERTICAL  DISPLACEMENTS 


Nevertheless,  they  do  not  fail  to 
show  the  tool-marks  of  epigene 
action — everywhere  escarpments 
are  retreating,  and  one  can  see 
that  already  vast  masses  of  rock 
have  been  removed  from  the  sur- 
face. The  accompanying  dia- 
gram (Fig.  64)  will  serve  to  give 
a  general  idea  of  the  geological 
structure  of  the  Basin  ranges. 
There  is  no  reason  to  believe 
that  the  crustal  movements  above 
referred  to  were  sudden  or  cata- 
strophic in  character.  Probably 
they  were  no  more  rapid  than 
those  which  have  affected  the 
plateau  of  the  Colorado. 

We  are  not  without  evidence 
of  similar  recent  dislocations  in 
the  Old  World,  and  there  as 
elsewhere  they  give  rise  to  more 
or  less  pronounced  surface-feat- 
ures. One  of  the  most  interest- 
ing examples  is  seen  in  the  great 
depression  that  extends  north- 
wards from  the  Gulf  of  Akabah 
by  the  Wady  el  Arabah,  the 
Dead  Sea,  the  valley  of  the  Jor- 
dan, and  Lake  Tiberias.  This 
long  hollow  would  appear  to 


§  --5 


160  EARTH  SCULPTURE 

have  come  into  existence  at  or  about  the  close  of 
Tertiary  times.  It  is  everywhere  bounded  by  normal 
faults  or  by  steep  monoclinal  folds,  the  one  kind  of 
structure  passing  into  the  other.  Before  this  depres- 
sion came  into  existence  the  region  it  now  traverses 
appears  to  have  been  a  broad  continuous  plateau, 
built  up  of  ancient  crystalline  and  Palaeozoic  rocks 
below,  and  approximately  horizontal  strata  of  Meso- 
zoic  age  above.  At  what  particular  date  this  plateau 
of  accumulation  first  appeared,  and  how  long  it  re- 
mained undisturbed,  we  cannot  tell.  Possibly  the 
movement  of  subsidence  to  which  the  Dead  Sea  owes 
its  origin  may  have  coincided  with  the  upheaval  that 
resulted  in  the  formation  of  the  plateau.  However 
that  may  have  been,  the  latter  was  eventually  tra- 
versed by  a  series  of  monoclinal  folds  and  parallel 
faults,  and  between  these  the  great  depression  of  the 
Jordan  came  into  existence.  The  Mesozoic  strata  of 
the  plateau  retain  their  approximately  horizontal  po- 
sition close  up  to  the  depression  along  its  eastern 
margin,  while  the  descent  from  the  west  is  much  less 
abrupt.  But  this  is  only  broadly  true.  When  the 
region  is  more  closely  investigated,  the  relatively  gen- 
tle dip  of  the  strata  along  the  west  side  of  the  depres- 
sion is  found  to  be  interrupted  again  and  again  by 
more  or  less  sharp  monoclinal  folds  and  by  normal 
faults,  the  presence  of  which  is  betrayed  at  the  sur- 
face by  corresponding  sudden  changes  in  the  form  of 
the  ground.  In  other  words,  the  descent  from  the 
plateau  on  the  west  is  often  by  a  series  of  broader 


VERTICAL  DISPLACEMENTS 


161 


and  narrower  terraces  and  escarp- 
ments, running  parallel  with  the 
trend  of  the  great  hollow.  The 
western  margin  of  the  Dead  Sea, 
for  example,  is  determined  by  a 
vertical  displacement,  similar  in 
character  to,  but  not  so  extensive 
as,  that  which  bounds  it  on  the 
east.  The  section  (Fig.  65)  will 
serve  to  illustrate  the  geological 
structures  referred  to. 

The  flexures  and  faults  of  this 
interesting  region  do  not  date 
beyond  the  close  of  the  Tertiary 
period,  and  consequently  there 
has  not  been  sufficient  time  to 
allow  of  a  complete  modification 
of  the  surface  by  epigene  action. 
The  most  conspicuous  features 
of  the  district  are  determined 
by  folds  and  fractures — under- 
ground structure  and  surface- 
configuration  to  a  large  extent 
coincide.  But  everywhere  also 
we  observe  the  evidence  of  ero- 
sion and  denudation.  Great 
sheets  of  rock  have  been  grad- 
ually removed  from  the  surface, 
which  is  seamed  and  scarred  by 
innumerable  ravines  and  water- 


tf 


1^1 


162  EARTH  SCULPTURE 

courses,  many  of  these  being  now  dry  and  deserted. 
According  to  Professor  Suess,  the  Jordan  depres- 
sion continues  north  between  the  Lebanon  and  the 
Anti-Lebanon,  through  the  valley  of  the  Nahr  el  Asi 
(the  Orontes)  to  near  Antioch.  The  same  geologist 
is  further  of  opinion  that  the  great  trough  of  the  Red 
Sea  and  most  of  the  lacustrine  hollows  of  East  Africa 
are  in  like  manner  due  to  direct  subsidence  of  the 
crust,  the  probability  being  that  they  and  the  Jordan 
depression  all  belong  to  one  and  the  same  system  of 
crustal  deformation.  It  is  noteworthy  that  the  de- 
pressed areas  of  Africa  lie  in  zones  or  belts  having  an 
approximately  meridional  direction,  that  they  are  not 
margined  or  surrounded  by  mountain-ranges,  but  are 
sunk  in  broad  plateaux,  and,  moreover,  are  accom- 
panied by  abundant  evidence  of  volcanic  action.  The 
troughs  are  mostly  broad,  and  vary  much  and  con- 
stantly in  height  above  the  sea,  so  that  they  are  obvi- 
ously not  the  result  of  erosion.  In  many  places  they 
are  flanked  on  both  sides  by  abrupt  declivities  com- 
parable in  character  to  those  that  overlook  the  Dead 
Sea.  In  some  cases,  however,  steep  bluffs  and  cliffs 
are  confined  to  one  side  of  a  depression  only.  In 
short,  we  have  in  East  Africa  the  same'  phenomena 
which  confront  us  in  Palestine.  The  earth's  crust  in 
all  those  regions  has  evidently  yielded  to  strain  or 
tension  by  snapping  across  and  subsiding.  In  place 
of  one  simple  normal  fault,  however,  we  see  a  com- 
plex system  of  parallel  dislocations  and  flexures,  the 
folded  and  shattered  rocks  having  settled  down  un- 


VERTICAL  DISPLACEMENTS  163 

equally,  while  molten  matter  and  loose  ejecta  issued 
here  and  there  in  less  or  greater  abundance  along 
the  chief  lines  of  rock-disturbance. 

Similar  geological  structures  on  a  smaller  scale 
may  be  seen  nearer  home,  and  are  well  exemplified 
in  the  region  of  the  Vosges  and  the  Black  Forest. 
These  opposing  mountains  are  the  counterparts  of 
each  other,  being  built  up  of  the  same  rocks,  arranged 
in  very  much  the  same  way.  The  basement  rocks 
are  granite  and  crystalline  schistose  rocks,  which  are 
overlaid  by  a  series  of  Mesozoic  strata.  In  the 
Vosges  the  dip  of  these  strata  is  westerly,  while  the 
corresponding  rocks  in  the  Black  Forest  are  inclined 
towards  the  east.  Between  the  two  ranges,  as  every- 
one knows,  lies  the  basin  of  the  upper  Rhine,  a  basin 
which,  like  that  of  the  Jordan,  has  been  determined 
by  a  number  of  parallel  normal  faults.  The  Meso- 
zoic strata  in  the  region  surrounding  the  two  ranges 
attain  a  thickness  of  at  least  5000  feet,  and  there  can 
be  no  doubt  that  these  originally  extended  from  west  to 
east  across  what  is  now  the  basin  of  the  Rhine.  This 
is  shown  by  the  simple  fact  that  the  strata  in  question 
occupy  that  basin.  (See  Fig.  66,  p.  164.)  Doubtless 
the  Mesozoic  rocks  were  originally  deposited  in  ap- 
proximately horizontal  positions.  Subsequently  the 
sea  retreated  from  the  area,  and  a  wide  land-surface— 
probably  an  elevated  plain  or  plateau — occupied  its 
place.  Eventually,  in  early  Tertiary  times,  the  region 
was  subjected  to  crustal  movements,  and  traversed 
from  south  to  north  by  a  series  of  dislocations,  with 


i64 


EARTH  SCULPTURE 


downthrows  in  opposite  di- 
rections. As  a  result  of  these 
displacements  the  Rhenish 
basin  came  into  existence, 
while  the  rock-masses  along 
its  margins  were  pushed  up 
to  form  the  ranges  of  the 
Vosges  and  the  Black 
Forest.  The  crustal  move- 
ments referred  to  appear  to 
have  been  continued  down  to 

3  §  post-Tertiary  times,  and 
have  probably  not  yet 

J£  ceased,  the  frequent  earth- 
Q  *~  quakes  experienced  in  the 
neighbourhood  of  Darm- 
stadt being  perhaps  an 
indication  of  progressive 
subsidence  along  lines  of 
dislocation.  It  is  interest- 

I  oo  ing  to  note  that  these  crustal 
movements  have  been  ac- 
companied from  time  to 
time  by  volcanic  action. 
The  well-known  Kaiserstuhl 
near  Freiburg,  for  example, 
is  the  skeleton  of  what  must 
have  been  a  very  consider- 
able volcano. 

The  evidence  that  subsid- 


VERTICAL  DISPLACEMENTS  165 

encc  in  the  Rhenish  basin  has  continued  into  the 
post-Tertiary  period  is  so  striking  that  it  may  be 
briefly  referred  to  here.  Deep  borings  have  shown 
that  the  Pleistocene  deposits  in  the  valley  of  the 
Rhine  in  Hesse  occupy  a  profound  hollow,  surrounded 
on  all  sides  by  older  rocks,  the  bottom  of  the  basin 
being  270  feet  deeper  than  the  lowest  part  of  its  rim 
at  Bingen.  These  deposits,  however,  are  not  lacus- 
trine, but  fluviatile.  Hence  we  must  infer  that  fluv- 
iatile  deposition  has  kept  pace  with  the  crustal 
movement.  As  the  bottom  of  the  Rhine  valley  has 
slowly  subsided,  the  river  has  flowed  on  without 
interruption,  continuously  filling  up  the  gradually 
deepening  basin  with  its  sediment.  This  is  only 
another  example  of  the  fact  that  movements  of  the 
crust,  whether  of  elevation  or  depression,  have  often 
>roceeded  so  slowly  that  they  have  been  unable  to 
lodify  the  direction  of  streams  and  rivers. 

While  we  recognise  the  influence  of  earth-move- 
Lents  in  determining  the  form  of  the  surface  in  the 
•egion  under  review,  it  is  obvious  that  much  rock- 
laterial  has  been  removed.  The  presence  of  the 
'esozoic  strata  in  the  basin  of  the  Rhine  shows  that 
:hese  must  formerly  have  extended  continuously  over 
:he  adjacent  tracts.  Yet  they  have  since  been  largely 
lenuded  away  from  the  higher  parts  of  the  Vosges 
ind  the  Black  Forest,  so  that  the  underlying  crystal- 
line rocks  have  been  laid  bare,  and  now  appear  at  the 
surface  over  considerable  areas. 

When  we  turn  our  attention  to  regions  of  highly 


i66  EARTH  SCULPTURE 

dislocated  rocks,  where  the  crustal  displacements  are 
of  much  greater  antiquity  than  those  we  have  just 
been  considering,  the  surface-features,  we  find,  have 
often  been  so  modified  by  denudation  that  the  posi- 
tion and  even  the  very  existence  of  normal  faults  can 
be  determined  only  by  close  observation.  In  other 
cases,  however,  they  give  rise  to  marked  features  at 
the  surface. 

The  following  section  across  a  portion  of  the  Lan- 
arkshire coal-field  is  drawn  upon  a  true  scale.  The 
section  traverses  several  normal  faults,  the  largest 


FIG.  67.    SECTION  OF  COAL-MEASURES  (ON  A  TRUE  SCALE)   NEAR 
CAMBUSNETHAN,  LANARKSHIRE. 

being  a  displacement  of  350  feet,  yet  there  is  no  feat- 
ure at  the  surface  to  indicate  its  presence. 

It  is  only  by  studying  the  geological  structure  that 
the  existence  of  such  dislocations  can  be  discovered. 
The  strata  of  the  region  in  question  are  of  much  the 
same  consistency  throughout,  and  have  therefore 
yielded  equally  to  the  various  agents  of  erosion. 
Thus  all  inequalities  of  surface  which  may  originally 
have  resulted  from  faulting  have  been  smoothed  out. 
It  is  doubtful,  however,  whether  such  relatively  small 
faults  ever  did  show  at  the  surface.  The  amount  of 
displacement  effected  by  them  usually  diminishes  up- 
wards, so  that  the  highest  coal-seams  are  hardly  dis- 


VERTICAL  DISPLACEMENTS  167 

located  to  such  an  extent  as  those  which  occur  at 
lower  levels.  Many  small  faults,  indeed,  die  out  up- 
wards altogether.  And  when  we  remember  that  the 
rocks  now  exposed  at  the  surface  were  formerly 
covered  by  enormous  sheets  of  strata  which  have 
since  been  removed  by  denudation,  it  is  not  hard 
to  believe  that  even  some  of  the  larger  faults  of 
our  coal-fields  may  actually  have  died  out  before 
the  original  surface  of  the  Carboniferous  strata  was 
reached. 

Some  normal  faults,  however,  are  so  very  extens- 
ive— the  amount  of  displacement  is  so  very  great — 
that  we  must  believe  they  did  reach  the  earth's  surface 
at  the  time  of  their  formation.  Yet  where  these 
faults  traverse  strata  having  much  the  same  charac- 
ter, they  produce  no  inequalities  of  level  at  the  sur- 
face. A  good  example  is  the  Tynedale  fault  of  the 
Newcastle  coal-field,  which  has  a  downthrow  in  some 
places  of  1 200  feet,  and  yet  its  existence  is  not  be- 
trayed by  the  configuration  of  the  ground.  (See 
Fig.  68,  p.  1 68.) 

Great  normal  faults,  however,  usually  do  show  more 
or  less  conspicuously  at  the  surface.  This  is  due  to 
the  fact  that  by  their  means  areas  of  soft  and  hard 
rock  are  often  brought  into  juxtaposition.  Many  ex- 
amples might  be  cited  from  Great  Britain.  Thus 
in  Scotland  the  Central  Lowlands,  consisting  largely 
of  relatively  soft  rocks,  have  been  brought  against 
the  harder  rocks  of  the  Highlands  on  the  one  hand, 
and  those  of  the  Southern  Uplands  on  the  other.  A 


i68  EARTH  SCULPTURE 

line  drawn  from  Stonehaven  in  a  south-west  direction 
to  the  Clyde  near  Helensburgh  is  at  once  the  geo- 
logical and  geographical  boundary  of  Highlands  and 
Lowlands,  while  a  similar  line  extending  from  Dun- 
bar  to  the  coast  of  Ayrshire  near  Girvan  forms  the 
corresponding  boundary  of  the  Lowlands  and  the 

GosforH*  S.E. 

Btirn. 


•IAIN        Co* 


FIG.  68.     SECTION  ON  A  TRUE  SCALE  ACROSS  "  TYNEDALE  FAULT," 
NEWCASTLE  COAL-FIELD. 

Southern  Uplands.  The  lines  in  question  are  great 
dislocations,  having  in  places  downthrows  of  5000  to 
6000  feet.  But  there  can  be  no  doubt  that  the  in- 
equalities at  the  surface  are  due  not  so  much  to  the 
amount  of  vertical  displacement  as  to  the  different 
character  of  the  rocks  on  opposite  sides  of  the  faults. 
This  is  well  shown  by  the  fact  that  the  disparity 
level  along  a  line  of  dislocation  varies  with  the  char- 


VERTICAL  DISPLACEMENTS  169 

acter  of  the  rocks  which  are  brought  into  juxtaposi- 
tion. Thus,  when  soft  sandstone,  as  in  Strathmore, 
abut  against  hard  crystalline  rocks,  the  latter  rise 
more  or  less  abruptly  above  the  former — the  line 
of  demarcation  between  Highlands  and  Lowlands  is 

S£. 


FIG.  69.    SECTION  ACROSS  GREAT  FAULT  BOUNDING  THE  HIGHLANDS 
NEAR  BlRNAM,  PERTHSHIRE. 

A,  "  hard  "  grits  and  shales  ;  j,  relatively  "  soft  "  sandstones,  etc.    Demarcation  between 
Highlands  and  Lowlands  well  marked. 

strongly  pronounced.  But  when,  as  between  the  val- 
leys of  the  Earn  and  the  Teith,  the  hard  igneous 
rocks  of  the  Lowlands  are  brought  against  the  crys- 
talline schists  of  the  Highlands,  the  geographical 
boundary  of  the  two  regions  is  not  nearly  so  well 
marked — the  Highland  mountains  seem  to  merge 
gradually  into  the  Lowland  hills.  And  the  same 
phenomena  are  conspicuously  displayed  along  the 
margin  of  the  Lowlands  and  the  Southern  Uplands. 
In  a  word,  it  is  obvious  that  while  the  position  of  the 
boundaries  that  separate  the  Lowlands  from  the 
mountain-areas  to  north  and  south  has  been  deter- 
mined by  normal  faults,  the  existing  configuration  is 
the  result  of  long-continued  and  profound  denudation. 
The  accompanying  sketch  sections  (Figs.  69,  70) 
will  serve  to  illustrate  the  foregoing  remarks. 


170 


EARTH  SCULPTURE 


Normal  faults,  as  we  have  seen,  have  often  deter- 
mined the  boundaries  between  lowlands  and  high- 
lands. Not  infrequently,  indeed,  it  can  be  shown 


ww. 


FIG.  70.    SECTION  ACROSS  GREAT  FAULT  BOUNDING  THE 
SOUTHERN  UPLANDS. 

A,  "  hard  "  greywackes,  etc.;  /",  "  hard  "  igneous  rocks  and  overlying  conglomerate  c. 
Demarcation  between  Uplands  and  Lowlands  not  well  marked. 

that  the  dominance  of  certain  mountains  is  due  rather 
to  the  sinking  down  of  adjacent  low-lying  tracts  than 
to  bulging  up  of  the  crust  within  the  mountain-areas 


B 


FIG.  71.     DIAGRAM  SECTION  ACROSS  HORSTGEBIRGE. 

<*,  granite,  gneiss,  etc.,  forming  the  "  Horst"  ;  £,  stratified  rocks  of  relatively  late  age,  resting 

upon  a,  dropped  down  along  lines  of  dislocation  ff;  0,  outlier  of  £,  showing  that 

the  strata  b  were  formerly  continuous  between  A  and  B. 

themselves.  Such  mountains  are,  of  course,  bounded 
by  faults,  and  are  known  to  German  geologists  as 
Horste  or  Rumpfgebirge,  the  Harz  being  a  good  ex- 
ample. The  Horste  of  Middle  Europe  are  composed 
for  the  most  part  of  crystalline  schists  and  Palaeozoic 
rocks,  more  or  less  highly  flexed  and  disturbed.  The 


VERTICAL  DISPLACEMENTS  171 

mountains  usually  rise  somewhat  suddenly  above  the 
surface  of  the  relatively  undisturbed  and  approxi- 
mately horizontal  Mesozoic  strata  of  the  adjacent  low 
grounds,  and  for  a  long  time  it  was  supposed  that 
these  strata  in  the  immediate  vicinity  of  the  Plorste 
were  littoral  deposits.  Such,  however,  is  not  the  case. 
They  are  of  relatively  deep-water  origin,  and,  before 
faulting  supervened,  may  have  covered  much  of  the 
high  lands  which  now  overlook  them.  It  is  obvious, 
in  short,  that  the  Horste  represent  portions  of  the  crust 
which  have  maintained  their  position  ;  they  are  mount- 
ains which  testify  to  a  former  higher  crustal  level ; 
the  surrounding  tracts  have  broken  away  from  them, 
and  dropped  to  a  lower  position. 

Probably  enough  has  now  been  advanced  to  show 
that  normal  faults  have  had  no  inconsiderable  share 
in  determining  surface-features.  This,  as  might  have 
been  expected,  is  most  conspicuous  in  regions  of  re- 
cent crustal  deformation  and  fracture,  where  epigene 
action  has  not  had  time  to  effect  much  modification. 
In  cases  of  very  ancient  fracture  and  displacement, 
lowever,  the  surface-features,  as  we  have  seen,  are 
y  greatly  modified,  and  if  well-marked  disparity  of 
level  is  still  often  met  with  along  lines  of  dislocation, 
this  is  mainly  due  to  the  fact  that  rocks  of  unequal 
endurance  have  been  brought  into  juxtaposition.  In 

case  of  very  considerable  displacement  it  will  usu- 
illy  happen,  indeed,  that  crystalline  schists,  plutonic 
rocks,  or  hard  Palaeozoic  strata  will  occur  upon  the 
ligh  side  and  relatively  softer  strata  on  the  low  side 


172  EARTH  SCULPTURE 

of  the  fault.  However  prolonged  and  intense  epigene 
action  may  have  been,  such  a  fault  will  nevertheless 
cause  a  marked  feature  at  the  surface,  so  long  as  the 
general  surface  of  the  land  remains  considerably  above 
the  base-level.  But  when  the  latter  is  approached 
denudation  will  eventually  cease  on  the  low  side  of 
the  fault,  while  material  will  continue  to  be  removed 
from  the  high  side,  and  the  disparity  between  the  two 
will  thus  tend  gradually  to  disappear.  In  short,  the 
irregularities  of  surface  determined  by  the  presence 
of  faults  pass  through  the  same  cycle  of  changes  as 
all  other  kinds  of  geological  structure.  Should  the 
base-level  remain  undisturbed  epigene  action  must 
eventually  reduce  every  inequality,  no  matter  what  its 
origin  may  have  been.  Again,  were  such  a  reduced 
land-surface  to  be  re-elevated  and  converted  into  a 
plateau,  the  lines  of  dislocation  that  happened  to 
separate  areas  of  hard  rock  from  regions  of  soft  rock 
would  once  more  determine  the  boundaries  between 
high  and  low  ground.  The  surface  of  the  soft  rocks 
would  be  lowered  most  readily,  while  the  more  durable 
hard  rocks  would  come  to  form  elevations.  In  a  word, 
the  features  that  obtained  before  the  land  was  reduced 
to  base-level  would,  under  the  influence  of  denudation, 
tend  to  re-appear. 


CHAPTER   VIII 

LAND-FORMS   DUE   DIRECTLY    OR   INDIRECTLY 
TO    IGNEOUS   ACTION 

PLUTONIC  AND  VOLCANIC  ROCKS — DEFORMATION  OF  SURFACE 
CAUSED  BY  INTRUSIONS — LACCOLITHS  OF  HENRY  MOUNTAINS 
— VOLCANOES,  STRUCTURE  AND  FORM  OF — MUD-CONES — GEY- 
SERS— FISSURE-ERUPTIONS  —  VOLCANIC  PLATEAUX  —  DENUD- 
ATION OF  VOLCANOES,  ETC.,  AND  RESULTING  FEATURES. 

IN  preceding  pages  we  have  had  frequent  occasion 
to  refer  to  igneous  rocks.  These,  as  we  have 
seen,  may  be  broadly  grouped  under  two  heads — Plu- 
tonic rocks  and  Volcanic  rocks.  The  former  have 
cooled  and  solidified  at  a  less  or  greater  depth  below 
the  surface  ;  the  latter,  on  the  other  hand,  have  been 
extruded  at  or  near  the  surface.  No  hard  and  fast 
line,  however,  can  be  drawn  between  these  two  groups. 
All  plutonic  rocks  are  indeed  intrusive — they  have 
solidified  below  ground ;  but  the  same  is  true  of  the 
sheets  and  dikes  which  traverse  a  volcano,  and  which, 
along  with  the  bedded  lavas  and  tuffs  they  traverse, 
are  properly  described  as  of  volcanic  origin.  It  will 
be  understood,  then,  that  the  term  plutonic  is  restricted 
to  intrusive  rocks  which  have  consolidated  at  rela- 
tively great  depths,  while  the  term  volcanic  includes 

173 


174  EARTH  SCULPTURE 

all  igneous  rocks  which  enter  or  have  entered  into 
the  formation  of  a  volcano,  or  which  have  evidently 
proceeded  from  any  focus  or  foci  of  eruption. 

It  is  needless  to  say  that  we  can  know  nothing  by 
direct  observation  of  the  conditions  and  phenomena 
which  attend  the  intrusion  of  deep-seated  plutonic 
rocks.  But  so  many  of  these  have  been  laid  bare  by 
denudation,  their  composition  and  their  relation  to  sur- 
rounding rock-masses  have  been  so  carefully  studied, 
that  geologists  have  learned  much  concerning  igneous 
action  of  which  but  for  denudation  they  must  have 
remained  largely  ignorant.  They  have  ascertained, 
for  example,  that  such  lavas  as  rhyolite,  andesite,  and 
basalt  have  their  deep-seated  equivalents  in  the  plu- 
tonic granites,  syenites,  and  gabbros.  That  is  to  say, 
we  know  that  the  same  molten  mass  solidifies  at  great 
depths  as  granite  or  other  wholly  crystalline  rock,  and 
at  the  surface  as  rhyolite  or  other  semi-crystalline  lava. 
In  short,  plutonic  rocks  and  their  volcanic  equivalents 
have  practically  the  same  chemical  composition.  An 
acid  lava  comes  from  an  acid  magma,  a  basic  lava  from 
a  basic  magma.  Hence  it  is  inferred  that  many  plu- 
tonic rocks  now  exposed  by  denudation  may  have  been 
the  deep-seated  sources  from  which  ancient  lavas  have 
proceeded.  On  the  other  hand,  there  is  reason  to 
believe  that  many  plutonic  masses  may  never  have 
had  any  such  volcanic  connections. 

But  whether  or  no  a  given  plutonic  mass  be  the 
deep-seated  source  of  some  long-vanished  volcano  or 
volcanoes  does  not  concern  us  here.  We  have  sim- 


LAND-FORMS  DUE   TO  IGNEOUS  ACTION    175 

ply  to  recognise  the  fact  that  its  exposure  at  the 
surface  is  the  direct  result  of  profound  denudation. 
Whether  its  intrusion  had  any  effect  in  deforming  the 
surface  we  cannot  tell.  Probably,  in  cases  where 
none  of  the  material  was  extruded  to  the  surface  by 
contemporaneous  volcanic  action,  there  may  have 
been  some  bulging  up  of  the  ground.  Deformation 
of  the  crust,  in  short,  may  quite  well  have  accom- 
panied the  subterranean  movements  of  great  masses 
of  molten  matter.  But  so  long  a  time  has  elapsed 
since  the  granites  and  other  highly  crystalline  plutonic 
rocks  were  intruded — so  enormous  has  been  the  thick- 
ness of  rock  removed  from  above  them — that  such 
intrusion  cannot  be  said  to  have  had  any  direct  effect 
in  the  production  of  existing  surface-features.  It  is 
quite  true  that  many  hills  and  mountains  are  com- 
posed largely  or  even  exclusively  of  plutonic  rocks ; 


5 

FIG.  72.     MOUNTAIN  OF  GRANITE. 

g,  granite  sending  veins  into  schists,  etc.,  O).     The  schists  have  been  more  readily 
lowered  by  erosion  than  the  granite. 

but  that  is  simply  owing  to  the  fact  that  these  rocks 
are  usually  more  durable  than  the  rocks  through 
which  they  rise.  When,  as  not  infrequently  happens, 
plutonic  masses  are  of  less  durable  consistency  and 


176  EARTH  SCULPTURE 

construction  than  the  rocks  that  surround  them,  the 
latter  invariably  dominate  and  overlook  the  former. 
Thus  while  granite  often  forms  prominent  mountains 
(Fig.  72,  p.  175),  not  infrequently  it  is  found  occupy- 
ing low  tracts  flanked  by  mountains  of  schist,  slate, 
or  other  rock.  (Fig.  73.) 


FIG.  73.     PLAIN  OF  GRANITE  OVERLOOKED  BY  MOUNTAINS  OF  SCHISTS,  ETC. 

f,  granite  ;  j,  schists,  etc.     The  granite  has  been  more  readily  lowered   by  erosion 
than  the  surrounding  schists. 

We  must  conclude,  then,  that  whatever  effect  may 
have  been  produced  at  the  surface  by  the  intrusion  of 
the  more  ancient  plutonic  rocks  of  England  and  other 
countries,  such  superficial  effects,  if  any,  have  long 
since  disappeared.  The  present  configuration  of  the 
ground  occupied  by  such  rocks  is  wholly  the  result  of 
epigene  action.  But  when  we  consider  the  phenomena 
of  more  recent  intrusions  of  igneous  rock,  we  find 
reason  to  conclude  that  these  have  not  only  had  a 
direct  effect  at  the  surface,  but  that  this  effect  has 
not  yet  in  all  cases  been  removed  by  denudation. 
The  ground  has  bulged  up,  and  the  swelling  of  the 
surface  is  still  conspicuous.  Among  the  most  re- 


LAND-FORMS  DUE  TO  IGNEOUS  ACTION    177 

markable  examples  known  are  the  laccoliths  or  lac- 
colites  (stone  cisterns)  of  the  Henry  Mountains 
(southern  Utah),  which  have  been  described  by  Mr. 
Gilbert.  In  that  region  molten  rock,  instead  of 
ascending  to  the  surface  and  building  up  mountains 
by  successive  eruptions,  has  stopped  at  a  lower  hori- 
zon, insinuated  itself  between  the  strata,  and  opened 
for  itself  a  chamber  by  lifting  all  the  superior  beds. 
(See  Fig.  74.)  Proceeding  from  a  laccolith  are  in- 


to. 74.  DIAGRAMMATIC  SECTION  OF  A  LACCOLITH  SHOWING  DOME-SHAPED 
OVATION  OF  SURFACE  ABOVE  THE  INTRUSIVE  ROCK.   (After  G.  K.  Gilbert.) 

/*,  pipe  or  conduit ;  sA,  sheet ;  d  d,  dikes. 

:rusions  of  the  same  kind  of  igneous  rock  (trachyte), 
some  of  which  (sheets)  have  squeezed  themselves  be- 
tween adjacent  beds,  while  others  (dikes)  traverse  the 
strata  at  less  or  greater  angles.  These  remarkable 
rocks  have  been  intruded  in  a  great  series  of  strata 
ranging  in  age  from  Carboniferous  to  Cretaceous, 
amongst  which  they  are  irregularly  distributed,  some 


178  EARTH  SCULPTURE 

appearing  in  the  Carboniferous,  some  in  the  Jura- 
Trias,  and  others  in  the  Cretaceous.  From  the  low- 
est to  the  highest  laccolith  the  range  is  not  less  than 
4000  feet,  those  which  are  above  not  infrequently 
overlapping  those  which  lie  below.  "  Their  horizon- 
tal distribution  is  as  irregular  as  the  arrangement  of 
volcanic  vents.  They  occur  in  clusters,  and  each 
cluster  is  marked  by  a  mountain.  In  Mount  Ellen 
there  are  perhaps  thirty  laccolites  ;  in  Mount  Holmes 
there  are  two  ;  and  in  Mount  Ellsworth  one.  Mount 
Pennell  and  Mount  Hillers  have  each  one  large  and 
several  small  ones."  The  highest  of  these  mountains 
attains  an  elevation  of  over  11,000  feet,  rising  some 
5000  feet  above  the  plateau  at  its  base.  The  strata 
of  which  that  plateau  is  built  up  are  approximately 
horizontal,  and  appear  at  one  time  to  have  been  cov- 
ered by  some  thousands  of  feet  of  Tertiary  deposits, 
the  nearest  remains  of  which  occur  at  a  distance  of 
thirty  miles  from  the  Henry  Mountains.  Mr.  Gilbert 
is  of  opinion  that  the  laccolites  were  most  probably 
intruded  after  the  deposition  of  the  Tertiary  strata, 
and  before  their  subsequent  removal  by  erosion. 

The  whole  structure  of  the  Henry  Mountains  shows 
that  the  actual  surface  was  affected  by  those  intru- 
sions, the  horizontal  strata  being  arched  upwards  so 
as  to  form  dome-shaped  elevations,  rising  prominently 
above  the  general  level  of  the  plateau.  The  laccoliths 
are  all  of  considerable  size,  the  smallest  measuring 
more  than  half  a  mile,  and  the  largest  about  four 
miles  in  diameter.  The  mountains  formed  by  them 


LAND-FORMS  DUE  TO  IGNEOUS  ACTION    179 

consist  of  a  group  of  five  individuals  separated  by  low 
passes,  but  having  no  definite  range  or  trend.  The 
subsequent  erosion  of  these  mountains,  Mr.  Gilbert 
remarks,  has  given  the  utmost  variety  of  exposure  to 
the  laccoliths.  In  some  places  these  are  not  yet  un- 
covered, and  we  see  only  the  arching  strata  which 
overlie  them,  the  strata  being  cut  across  by  only  a 
few  dikes  or  traversed  by  a  network  of  dikes  and 
sheets.  In  other  places  denudation  has  partly  bared 
the  laccoliths  or  even  completely  exposed  them,  so 
that  their  original  form  can  be  seen.  In  yet  other 
places  the  bared  laccolith  itself  has  been  attacked  by 
the  elements,  and  its  original  form  more  or  less 
changed.  It  is  even  quite  possible  that  occasionally 
laccoliths  may  have  been  entirely  demolished,  and 
that  some  of  the  truncated  dikes  now  visible  at  the 
surface  may  mark  the  old  fissures  or  conduits  through 
which  such  vanished  laccoliths  were  injected. 

From  the  evidence  just  referred  to,  it  is  obvious 
that  intrusions  of  igneous  rock,  if  of  sufficient  thick- 
ness, are  capable  of  warping  the  surface,  and  of  form- 
ing more  or  less  considerable  elevations.  But  as 
erosion  tends  to  reduce  all  such  upheavals  more  or 
less  rapidly,  it  is  only  those  of  relatively  recent  age 
that  can  retain  any  trace  of  their  original  configura- 
tion. All  masses  of  intrusive  rock  of  great  geological 
antiquity,  which  now  form  hills  and  mountains,  do  so 
in  virtue  of  their  greater  resistance  to  the  action  of 
epigene  agents.  They  may  have  arched  up  the  rocks 
underneath  which  they  formerly  lay  buried,  and  so 


1 80  EAR  TH  SCULP  TURE 

produced  more  or  less  prominent  elevations  at  the 
surface,  but  such  primeval  land-forms  have  been  en- 
tirely removed — the  features  now  visible  are  the 
direct  result  of  erosion  and  denudation. 

Of  true  volcanic  rocks  it  is  not  necessary  to  say 
much.  Their  eruption  at  and  near  the  surface  gives 
rise  to  hills  and  mountains  of  accumulation,  the  gen- 
eral aspect  and  structure  of  which  are  sufficiently  fa- 
miliar. The  typical  volcano  is  a  truncated  cone,  built 
up  usually  of  successive  lava-flows  and  sheets  of  loose 
ejecta.  At  the  summit  is  the  central  cup,  or  crater, 
marking  the  site  of  the  vertical  funnel,  or  throat, 
through  which  the  various  volcanic  products  find 
passage  to  the  surface.  These  are  naturally  arranged 
round  the  focus  of  eruption  in  a  series  of  irregular 
sheets,  beds,  and  heaps,  which  dip  outwards  in  all 
directions.  It  is  this  disposition  of  the  materials 
which  gives  its  characteristic  form  to  a  volcano.  The 
upper  part  of  the  cone  inclines  at  an  angle  of  30°  to 
35°,  but  this  steep  slope  gradually  decreases  until 
towards  the  base  the  inclination  may  not  exceed  3° 
or  5°.  In  a  typical  volcano,  therefore,  the  internal 
geological  structure  and  the  external  configuration 
coincide — the  mountain  with  its  graceful  outline  is 
the  direct  result  of  subterranean  action.  It  is  obvi- 
ous, however,  that  the  quaquaversal  arrangement  of 
the  lavas  and  tuffs  is  a  weak  structure.  Many  cones, 
it  is  true,  are  braced  and  strengthened  by  dikes  and 
other  protrusions  of  molten  rock,  which  consolidate  in 
the  cracks  and  fissures  that  often  traverse  a  volcanic 


LAND-FORMS  DUE  TO  IGNEOUS  ACTION    181 

mountain  in  all  directions.  But,  although  such  in- 
trusions may  delay,  they  cannot  prevent  the  ultimate 
degradation  of  a  volcano  which  has  ceased  to  be 
active. 

Active  and  dormant  or  recently  extinct  volcanoes 
differ  in  form,  to  some  extent,  according  to  the  pre- 
valent character  of  their  constituent  rocks,  and  the 
manner  in  which  these  have  been  heaped  up.  Some 
cones  consist  of  cinders,  or  other  fragmental  ejecta, 
with  which  no  lava  may  be  associated.  Not  infre- 
quently, again,  such  cones  have  given  vent  to  one 
or  more  lava-flows.  From  small  cinder-cones,  show- 
ing a  single  coutie,  to  great  volcanoes  built  up  of  a 
multitudinous  succession  of  lavas  and  sheets  of  frag- 
mental materials,  there  are  all  gradations.  The 
smaller  cones  are  often  the  products  of  a  single 
eruption  ;  while  the  larger  cones  owe  their  origin 
to  many  successive  eruptions,  between  some  of  which 
there  may  have  been  prolonged  periods  of  apparently 
complete  repose.  The  beautiful  symmetry  of  the 
typical  cone  is  often  disturbed.  This  is  due  some- 
times to  the  shifting  of  the  central  focus  of  eruption  ; 
sometimes  to  the  escape  of  lava  and  ejecta  from 
lateral  fissures  opening  on  the  slopes  of  the  mountain. 
Not  infrequently,  also,  the  symmetry  of  a  growing 
cone  is  liable  to  modification  by  the  action  of  the 
prevalent  wind,  the  loose  ejecta  during  an  eruption 
falling  in  greatest  bulk  to  leeward. 

Tuff-cones  and  cinder-cones  range  in  importance 
from  mere  inconsiderable  hills  to  mountains  approach- 


1 82  EARTH  SCULPTURE 

ing  or  exceeding  1000  feet  in  height.  In  the  typical 
cinder-cone  the  crater  is  small  in  proportion  to  the 
size  of  the  volcano  ;  it  is  simply  an  inconsiderable 
depression  at  the  summit  of  the  cone.  Occasionally, 
however,  we  meet  with  large  crateral  hollows,  mostly 
now  occupied  by  lakes  ringed  round  by  merely  an 
insignificant  ridge  of  fragmental  materials.  Some- 
times, indeed,  such  large  hollows  show  no  enveloping 
ring  whatsoever.  Extensive  craters*  of  this  kind  are 
believed  to  be  the  result  of  explosive  eruptions,  and 
it  is  quite  possible,  or  even  probable,  that  their  width 
has  been  considerably  increased  by  subsequent  cav- 
ing in  of  the  ground.  Cinder-cones  and  tuff-cones 
vary  in  form  according  to  the  character  of  their  con- 
stituent materials.  When  coarse  slags  and  scoria? 
or  pumice  predominate,  the  sides  of  the  cone  may 
have  an  inclination  of  35°,  or  even  of  40°.  When 
the  materials  are  not  quite  so  coarse,  the  angle  of 
slope  is  not  so  great ;  it  diminishes,  in  short,  as  the 
ejecta  become  more  finely  divided,  so  as  sometimes 
not  to  exceed  15°. 

Just  as  there  are  cones  composed  chiefly  or  exclu 
ively  of  fragmental  materials,  so  there  are  volcano 
built  up  of  one  or  of  many  successive  lava-flows,  with 
which  loose  ejecta  may  be  very  sparingly  associated, 
or  even  sometimes  absent  altogether.  Lava-cones 
likewise  vary  in  shape  and  size  according  to  the 
nature  of  their  component  rocks.  Some  form  abrupt 
hills  of  no  great  height ;  while  others  are  depressed 
cones,  attaining  a  great  elevation  and  sloping  at  a 


es 

: 


LAND-FORMS  DUE  TO  IGNEOUS  ACTION     183 

very  small  angle,  so  as  to  occupy  wide  tracts.  The 
abrupt  cones  consist  chiefly  of  the  more  viscous  lavas 
which  have  coagulated  immediately  round  the  focus 
of  eruption.  The  depressed  cones,  on  the  other 
hand,  are  built  up  of  the  more  liquid  lavas,  which 
flow  out  rapidly,  and  reach  relatively  greater  distances 
from  the  focus  of  eruption.  Not  infrequently  the 
cones  formed  by  the  out  welling  of  very  viscid  lava 
show  no  crater — the  lava  coagulates  around  and 
above  the  vent.  In  other  cases  the  top  of  the  abrupt 
dome-shaped  cone  is  blown  out  by  escaping  gases, 
and  a  crater-shaped  hollow  is  thus  formed.  The 
volcanoes  of  the  Hawaiian  Islands  present  the  grand- 
est examples  of  the  eruption  of  liquid  lavas.  Hawaii 
itself  is  made  up  of  five  volcanic  mountains,  ranging 
in  height  from  some  4000  feet  up  to  nearly  14,000 
feet.  All  these  are  depressed  cones.  Mauna  Loa 
(13,675  feet),  for  example,  has  a  broad,  flattened 
summit,  sunk  in  which  is  the  great  cauldron-like 
crater,  some  3^  miles  in  length  by  \\  in  width,  and 
800  feet  deep.  From  the  lip  of  this  crater  the  mount- 
ain slopes  outwards  at  an  angle  of  3°,  which  gradu- 
ally increases  to  7°,  the  diameter  of  the  mountain 
at  its  base  being  not  less  than  30-40  miles. 

But  composite  cones,  built  up  of  lava  and  loose 
ejecta,  are  of  far  more  common  occurrence  than 
cones  composed  of  lava  alone.  To  this  class  belong 
most  of  the  better-known  volcanic  mountains.  Their 
general  characters  have  already  been  outlined  in  the 
short  description  we  have  given  of  a  typical  volcano. 


1 84  EARTH  SCULPTURE 

It  remains  to  be  noted  that  many  composite  vol- 
canoes show  a  cone-in-cone  structure.  During  some 
paroxysmal  eruption  the  upper  portion  of  a  volcano 
may  be  destroyed — shattered  and  blown  into  frag- 
ments. Or,  as  a  result  of  long-continued  activity, 
the  mountain  becomes  partially  eviscerated,  and  the 
upper  part  of  the  cone  eventually  caves  in,  and  a  vast 
cauldron  is  formed,  after  which  a  protracted  period 
of  repose  may  ensue.  When  the  volcanic  forces 
again  come  into  action  a  younger  cone,  or  it  may 
be  several  such  cones,  gradually  grow  up  within  the 
walls  of  the  old  crater.  The  younger  cones  may 
rise  in  the  middle  of  the  great  hollow,  or  they  may 
be  eccentric,  as  in  the  case  of  Vesuvius,  which  has 
grown  up  upon  the  rim  of  the  large  crater  of  Monte 
Somma. 

Of  comparatively  little  importance  from  our  pre- 
sent point  of  view  are  mud-volcanoes.  Some  of  these 
owe  their  origin  to  the  escape  of  steam  and  hot 
water  through  disintegrated  and  decomposed  volcanic 
materials,  either  tuff  or  lava,  or  both.  They  are 
usually  of  inconsiderable  size,  many  being  mere 
monticles,  while  others  may  exceed  100  feet  in  height. 
They  show  craters  atop,  and  have  the  general  form 
of  tuff-cones.  Their  origin  is  obvious.  The  mud  is- 
simply  flicked  out  as  it  bubbles  and  sputters,  and  the 
material  thus  accumulates  round  the  margins  of  the 
cauldron,  until  a  cone  is  gradually  built  up.  Other 
so-called  mud-volcanoes  have  really  no  connection 
with  true  volcanic  action,  but  owe  their  origin  to  the 


LAND-FORMS  DUE  TO  IGNEOUS  ACTION    185 

continuous  or  spasmodic  escape  of  various  gases, 
such  as  marsh-gas,  carbonic  acid,  sulphuretted  hydro- 
gen, etc.  The  mud  of  which  they  are  chiefly  com- 
posed is  saline,  and  usually  cold.  Now  and  again, 
however,  stones  and  debris  may  be  ejected.  These 
"volcanoes"  (variously  known  as  salses,  air-volcanoes, 
and  maccalubas)  usually  form  groups  of  conical  hill- 
ocks like  miniature  volcanic  cones.  Here  also  may 
be  noted,  in  passing,  the  sinter-cones  formed  by  those 
eruptive  fountains  of  hot  water  and  steam  which  are 
known  under  the  general  term  of  geysers.  When 
the  geyser  erupts  on  level,  or  approximately  level, 
ground,  the  sinter  tends  to  assume  a  dome-shape  ; 
when,  on  the  other  hand,  the  springs  escape  upon  a 
slope,  the  silicious  deposits  are  not  infrequently  ar- 
ranged in  successive  terraces. 

All  the  volcanic  eruptions  to  which  we  have  been 
referring  have  proceeded  from  isolated  foci.  Some 
volcanoes  are  quite  solitary,  others  occur  in  irregular 
groups,  while  yet  others  appear  at  intervals  along  a 
given  line.  These  last  are  obviously  connected  with 
great  rectilinear  or  curved  dislocations  of  the  earth's 
crust ;  not  a  few  of  the  former,  however,  apparently 
indicate  the  sites  of  funnels  or  pipes  which  have  been 
simply  blasted  out  by  the  escape  of  elastic  vapours. 
There  is  yet  another  class  of  volcanic  eruptions  which 
have  played  a  prominent  part  in  geological  history, 
although  they  are  not  now  so  common.  These  are  the 
fissiire  or  massive  eruptions,  of  which  the  best  ex- 
amples at  the  present  time  are  furnished  by  Iceland. 


1 86  EARTH  SCULPTURE 

Lavas,  usually  of  the  more  liquid  kind,  well  out  some- 
times simultaneously  from  more  or  less  numerous 
vents  situated  upon  lines  of  fracture,  or  from  the 
lips  of  the  fissures  themselves.  Usually  such  floods 
and  deluges  of  lava  are  not  accompanied  by  the  dis- 
charge of  any  fragmental  materials.  Sheet  after 
sheet  of  molten  rock  has  been  discharged  in  this 
manner  so  as  to  completely  bury  former  land-surfaces, 
filling  up  valleys,  submerging  hills,  and  eventually 
building  up  great  plains  and  plateaux  of  accumula- 
tion. The  basalt-plains  of  Western  North  America, 
which  occupy  a  larger  area  than  France  and  Great 
Britain,  are  the  products  of  such  massive  eruptions, 
the  lavas  reaching  an  average  thickness  of  2000  feet. 
The  older  basalts  of  Iceland,  the  Faroe  Islands,  the 
Inner  Hebrides,  and  Antrim  are  the  relics  of  similar 
vast  fissure  eruptions.  And  of  like  origin  are  the 
basaltic  plateaux  of  Abyssinia  and  the  Deccan  in 
India.  The  volcanic  phenomena  of  the  Hawaiian 
Islands  have  also  much  in  common  with  fissure  or 
massive  eruptions. 

The  forms  assumed  by  the  materials  accumulated 
at  the  surface  by  subterranean  action  are  all  more  or 
less  distinctive  and  characteristic.  Hills,  mountains, 
plains,  and  plateaux,  which  owe  their  origin  directly 
to  volcanic  activity,  agree  in  this  respect,  that  their 
internal  structure  and  external  form  coincide.  Even 
the  most  perfectly  preserved  examples  of  volcanic  ac- 
cumulation, however,  are  seldom  without  some  trace 
of  the  modifying  influence  of  epigene  action.  The 


LAND-FORMS  DUE  TO  IGNEOUS  ACTION     187 

shape  of  a  volcanic  cone,  for  example,  during  its 
period  of  growth  is  subject  to  modification.  Wind 
affects  the  distribution  of  loose  ejecta,  while  rain  and 
torrents  sweep  down  materials,  and  gullies  and  ravines 
furrow  the  slopes  of  the  mountain.  The  ravages 
thus  caused  continue  to  be  repaired  from  time  to 
time  so  long  as  the  volcano  remains  active.  But 
when  its  fires  die  out  and  the  mountain  is  given  over 
to  the  undisputed  power  of  the  epigene  agents,  the 
work  of  degradation  and  decay  proceeds  apace.  The 
rate  of  this  inevitable  destruction  is  influenced  by 
many  circumstances — by  the  nature  and  structure  of 
the  materials,  for  example,  and  the  character  of  the 
climate.  Thus,  cones  built  up  of  loose  scoriae  are 
likely  to  endure  for  a  longer  time  than  cones  com- 
posed of  fine  tuff  and  hardened  mud.  Rain  falling 
upon  the  former  is  simply  absorbed,  and  consequently 
no  torrents  scour  and  eat  their  way  into  the  flanks  of 
the  cones,  while  tuff-  and  mud-cones  are  more  or  less 
rapidly  washed  down  and  degraded.  Again,  a  com- 
posite volcanic  mountain  of  complicated  structure, 
the  product  of  several  closely  associated  vents,  but- 
tressed and  braced  by  great  pipes  of  crystalline  rock 
and  an  abundant  series  of  larger  and  smaller  dikes, 
is  better  able  to  withstand  the  assaults  of  epigene 
agents  than  a  cone  of  simpler  build.  Sooner  or  later, 
however,  even  the  strongest  volcanic  mountain  must 
succumb.  Constantly  eaten  into,  sapped,  and  under- 
mined, it  will  eventually  be  levelled. 

In  regions  of  extinct  volcanoes  we  may  study  every 


i88 


EARTH  SCULPTURE 


stage  in  the  process  of  demolition.  Isolated  cones 
and  groups  of  cones  crumble  away,  until  all  the  lavas 
and  tuffs  ejected  from  the  old  vents  may  have  disap- 
peared, and  the  only  evidence  of  former  volcanic 
action  that  may  remain  are  the  basal  portions  of  the 
dikes  that  proceeded  from  the  foci,  and  the  solid 
cores  with  which  the  latter  were  finally  plugged  up. 
(See  Fig.  75.)  As  these  cores  usually  consist  of  more 


FIG.  75.    VIEW  OF  NECKS  =  CORES  OF  OLD  VOLCANOES.     (Powell.) 

durable  materials  than  the  rocks  they  pierce,  they 
tend  to  form  somewhat  abrupt  conical  hills.  It  goes 
without  saying  that  such  extreme  cases  of  denudation 
are  met  with  only  in  regions  where  volcanic  action 
has  for  a  long  time  been  extinct.  Excellent  exam- 


LAND-FORMS  DUE  TO  IGNEOUS  ACTION     189 

pies  on  a  relatively  small  scale  are  furnished  by  the 
so-called  "  Necks"  of  Scotland,  of  which  the  accom- 
panying section  (Fig.  76)  shows  the  general  phe- 
nomena. Similar  structures  occur  in  many  parts  of 
Europe  and  North  America. 

Mini*  Hill 


s 

FIG.  76.    SECTION  OF  HIGHLY  DENUDED  VOLCANO.    MINTO  HILL, 
ROXBURGHSHIRE. 

.A^,  throat  or  neck  of  volcano  plugged  up  with  ejectamenta,  angular  and  subangular  stones, 
grit,  dust,  etc. ;  S,  Silurian  rocks  ;  Z>,  Old  Red  Sandstone  strata. 

Frequently  the  products  of  great  volcanic  eruptions 
of  vast  geological  antiquity  have  been  largely  pre- 
served, owing  to  their  subsequent  burial  under  sedi- 
mentary accumulations.  Many  of  the  hill-ranges  of 
Central  Scotland,  for  example,  are  built  up  of  lavas 
and  tuffs.  These  are  the  relics  of  volcanoes  which 
came  into  existence  in  Palaeozoic  times,  and  after 
erupting  molten  and  fragmental  materials  for  longer 
or  shorter  periods,  eventually  died  out,  becoming  sub- 
merged and  covered  with  sedimentary  accumulations 
to  depths  of  several  thousand  feet.  Subsequent  ele- 
vation of  the  region  brought  these  sediments  under 
the  operation  of  the  agents  of  erosion,  and  in  time 
great  thicknesses  were  removed,  so  that  ultimately 
the  ancient  volcanic  rocks  were  again  laid  bare  and 
in  their  turn  exposed  to  denudation.  But  if  the  lat- 


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EARTH  SCULPTURE 


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ter  now  form  hills,  it  is  simply  be- 
cause they  consist  for  the  most  part 
of  more  durable  rock  than  the  form- 
ations amongst  which  they  lie. 
It  is  needless  to  say  that  all  trace 
of  their  original  configuration  has 
disappeared.  Indeed  that  had  al- 
ready vanished  before  the  extinct 
volcanoes  became  entombed.  Now 
and  again  the  sites  of  the  old  foci 
of  eruption  seem  to  be  indicated  by 
bosses  and  dikes  of  intrusive  rock, 
but  the  general  form  and  aspect  of 
the  hills  are  solely  the  results  of 
erosion,  determined  and  guided  by 
geological  structure  and  the  nature 
and  character  of  the  old  volcanic 
materials.  They  are  true  hills  of 
circumdenudation.  (See  Fig.  77.) 
The  massive  or  fissure  eruptions 
of  former  times  have  in  like  man- 
ner been  largely  modified  by  subse- 
quent epigene  action.  Although 
some  of  these  belong  to  a  compar- 
atively recent  geological  period, 
they  have  yet  been  so  carved  and  cut 
up,  that  their  original  plateau- 
character  has  become  obscured  or 
even  lost.  Yet  there  can  be  no 
doubt  that  they  formerly  existed 


LAND-FORMS  DUE  TO  IGNEOUS  ACTION     191 

as  broad  plains  and  plateaux,  occupying  many  thou- 
sands of  square  miles.  The  older  hills  of  Iceland,  all 
the  Faroe  Islands,  and  the  basalt  hills  of  the  Inner 
Hebrides  and  Antrim  are  the  relics  of  vast  plateaux, 
which  were  all  probably  at  one  time  connected.  The 
general  aspect  of  the  hills  carved  out  of  such  plateaux 
is  well  illustrated  by  the  Faroe  Islands,  to  which  some 
reference  has  been  made  in  Chapter  III. 

It  is  believed,  as  already  mentioned,  that  massive 
eruptions  have  proceeded  rather  from  systems  of  fis- 
sures than  from  separate  and  individual  foci,  after  the 
manner  of  most  modern  volcanoes  of  the  cone  and 
crater  type.  During  the  eruption  of  the  plateau-basalts 
of  Antrim  and  the  Inner  Hebrides,  molten  rock 
underlay  not  only  those  regions,  but  wide  areas  be- 
yond, in  the  north  of  Ireland  and  through  out  cen- 
tral and  southern  Scotland  and  the  north  of  Eng- 
land. All  these  areas  are  traversed  by  dikes  of 
basalt,  which  become  more  and  more  abundant  as 
they  are  followed  towards  the  regions  occupied  by 
the  basalt-flows.  It  is  from  these  dikes  that  the 
latter  appear  to  have  proceeded.  From  the  dikes 
that  are  now  seen  striking  across  Scotland  and  the 
north  of  England  probably  no  outflow  of  lava  took 
place  ;  the  fissures  up  through  which  the  molten  rock 
came  did  not  in  those  regions  reach  the  surface. 
They  are  now  exposed  simply  owing  to  denudation. 
Not  a  few  dikes  indeed  still  lie  concealed.  In  the  coal- 
fields these  are  found  cutting  across  the  lower  seams, 
but  wedging  out  before  the  upper  seams  are  reached. 


192  EARTH  SCULPTURE 

The  larger  dikes  in  central  Scotland  often  form 
conspicuous  objects  in  a  landscape.  Owing  to  the 
superior  durability  of  the  basalt,  they  rise  above  the 
surface  of  the  sedimentary  rocks  they  traverse,  and 
may  occasionally  be  followed  for  miles,  running  as 
they  do  like  great  walls  or  prominent  ridges  across 
dale  and  hill.  As  examples  may  be  cited  two  large 
parallel  dikes  which  may  be  traced  for  many  miles 
from  Friarton  Hill,  near  Perth,  in  a  westerly  direc- 
tion. Near  Dupplin,  the  more  northerly  of  the  two 
gives  rise  to  a  long  prominent  bank,  which  is  fol- 
lowed for  some  miles  by  an  old  Roman  road.  In 
the  neighbourhood  of  Crier!  both  dikes  are  equally 
conspicuous,  rising  as  bold  wall-like  ridges,  the  more 
prominent  of  the  two  forming  the  steep  crag  upon 
which  Drummond  Castle  is  perched.  When  dikes 
cut  through  rocks  as  durable  as  themselves  they 
cease  to  produce  any  marked  feature  at  the  surface. 
On  the  other  hand,  when  the  rocks  traversed  by 
them  are  the  most  resistant,  the  presence  of  the  dikes 
is  indicated  by  long  trenches  or  hollows  at  the  surface. 
Nothing  could  be  so  impressive  and  suggestive  of  the 
potency  of  long-continued  erosion  than  the  cropping 
out  of  these  remarkable  dikes.  Their  intrusion 
appears  to  have  taken  place  in  Tertiary  times,  and 
the  great  majority  of  those  which  occur  in  the  main- 
land of  Britain  never  actually  communicated  with 
the  surface  at  the  time  of  their  formation.  They 
cooled  and  consolidated  below  ground,  yet  we  now 
see  them  laid  bare  not  only  in  the  low  grounds  and 


LAND-FORM$~mJE  TO  IGNEOUS  ACTION    193 


in  valleys,  but  upon  hill-slopes  and  hill-tops.  Obvi- 
ously hundreds  of  feet  of  rock  have  been  removed 
from  the  whole  land-surface  since  those  dikes  were 
injected. 

In  fine,  then,  we  conclude  that  many  most  con- 
spicuous and  characteristic  features  of  the  land  owe 
their  origin  to  igneous  action.  In  some  places  the 
intrusion  of  masses  of  molten  rock  has  produced 
more  or  less  prominent  swelling  and  bulging  at  the 
surface,  while  the  outpouring  of  volcanic  materials 
has  resulted  in  the  formation  of  hills  and  mountains, 
and  of  plains  and  plateaux  of  accumulation.  Ere 
long,  however,  all  such  land-forms  become  modified 
by  epigene  action,  and  more  or  less  completely 
changed.  Intrusive  masses  formerly  deeply  buried 
are  eventually  exposed,  and,  owing  to  the  more  rapid 
removal  of  the  rocks  through  which  they  rise,  may 
come  to  form  mountains  of  circumdenudation,  while 
these  in  their  turn  tend  to  be  reduced  to  a  base-level. 
Volcanoes,  in  like  manner,  are  broken  down  and 
crumble  away,  until  it  may  be  the  only  relics  that 
remain  are  plugged-up  vents,  and  the  dikes  proceed- 
ing from  them,  every  fragment  of  the  cones  having 
vanished.  Or  the  lavas  of  former  times,  having 
been  interbedded  with  and  deeply  buried  under  strata 
of  aqueous  formation,  may,  owing  to  their  superior 
durability,  come  to  form  escarpment-hills  and  mount- 
ains, when  the  strata  originally  deposited  above 
them  have  been  removed  by  denudation.  So  again 
volcanic  plateaux  are  dug  into  by  erosion,  and  pass 


194 


EARTH  SCULPTURE 


through  a  well-marked  cycle  of  changes.  The  plat- 
eaux are  broken  up  into  groups  of  pyramidal  mount- 
ains, and  these  in  time  are  reduced,  and  may  even 
be  entirely  replaced  by  plains  of  erosion.  Thus  in 
lands  which  have  been  for  long  periods  of  time  ex- 
posed to  erosion,  although  evidence  of  former  igneous 
action  may  abound,  and  irruptive  and  eruptive  rocks 
may  enter  prominently  into  the  formation  of  the 
more  striking  surface-features,  the  shape  of  the  latter 
we  see  is  entirely  the  result  of  denudation  and  erosion. 
If  the  igneous  rocks  now  form  hills  and  mountains, 
it  is  because  of  their  superior  durability.  Intrusive 
and  effusive  rocks  alike  appear  at  the  surface,  and 
the  forms  they  assume  depend  chiefly  upon  the  geo- 
logical structure  and  mineralogical  character  of  the 
masses. 


CHAPTER  IX 

INFLUENCE  OF  ROCK  CHARACTER  IN  THE 
DETERMINA  TION  OF  LAND-FORMS. 

JOINTS  IN  ROCKS  AND  THE  PART  THEY  PLAY  IN  DETERMINING 
SURFACE-FEATURES TEXTURE  AND  MINERALOGICAL  COM- 
POSITION OF  ROCKS  IN  RELATION  TO  WEATHERING — FORMS 
ASSUMED  BY  VARIOUS  ROCKS. 

THE  origin  of  surface-features,  as  we  have  now 
learned,  is  frequently  complex.  Only  in  very 
few  cases  can  we  assert  that  any  prominent  feature 
is  the  direct  result  of  crustal  movement  alone.  In 
time  all  features  due  to  plutonic  or  subterranean 
action  become  more  or  less  modified.  We  are  justi- 
fied in  maintaining  that  the  great  mountain-chains 
of  the  globe  owe  their  origin  indeed  to  folding  and 
fracturing  of  the  crust ;  but  even  the  youngest  of 
these  has  yet  been  so  profoundly  modified  by  epigene 
action,  that  the  external  configuration  no  longer 
coincides,  save  in  a  general  way,  with  the  internal 
geological  structure.  Each  chain  as  a  whole  owes 
its  existence  to  crustal  deformation,  but  the  individual 
mountains  of  which  it  consists  are  largely  monuments 
of  erosion.  And  so  of  land-surfaces  generally  we 
may  say  that  their  more  prominent  features  are  the 

195 


196  EARTH  SCULPTURE 

result  of  denudation,  guided  and  controlled  by  geo- 
logical structure.  We  cannot  study  the  configuration 
of  the  land,  however,  without  perceiving  that  the 
relative  durability  of  rocks  has  also  had  some  share 
in  determining  the  form  of  the  surface.  In  regions 
composed  largely  of  "  soft "  rocks  we  may  note  a 
general  absence  of  abrupt  and  broken  outlines ;  the 
surface  even  when  hilly  is  usually  rounded  and  gently 
undulating.  It  is  otherwise  when  "hard"  rocks  pre- 
dominate, the  features  assumed  by  these  tending  to 
be  less  smooth  and  flowing.  The  surface  becomes 
more  diversified  still,  however,  when  both  soft  and 
hard  rocks  occur  together.  In  a  word,  hard  rocks  at 
all  elevations  offer  most  resistance,  while  soft  rocks 
more  readily  succumb  to  epigene  action.  We  thus 
arrive  at  the  general  conclusion  that  the  form  assumed 
by  the  land  under  long-continued  erosion  and  denud- 
ation is  determined  directly  by  the  character  of  the 
rocks  and  the  mode  of  their  arrangement,  and  in- 
directly, of  course,  by  igneous  action  and  crustal 
movements,  to  which  the  most  striking  and  conspicu- 
ous geological  structures  are  due. 

These  general  conclusions  have  now  been  suffi- 
ciently illustrated,  and  we^may  next  consider  certain 
surface-features  a  little  more  closely.  Rocks,  as  we 
have  seen,  consist  roughly  of  two  great  classes — those 
which  occur  in  more  or  less  distinct  beds  or  strata,  and 
those  which  show  no  such  arrangement,  but  appear  as 
amorphous  masses.  The  former  class  is  typically 
represented  by  sandstones,  shales,  and  limestones,  the 


INFLUENCE  OF  ROCK  CHARACTER     197 

latter  by  granite,  syenite,  and  other  eruptive  rocks. 
Most  of  the  bedded  rocks  are  fragmental  or  clastic  ; 
but  crystalline  rocks,  such  as  the  various  lavas,  not  in- 
frequently assume  bedded  forms.  With  few  excep- 
tions all  great  amorphous  rock-masses  are  crystalline. 
There  is  yet  another  important  group  of  crystalline 
rocks — the  schists — which  to  some  extent  simulate 
the  characteristic  structures  of  clastic  rocks.  Thus 
they  often  show  a  kind  of  bedding,  and  their  foliation 
mimics,  as  it  were,  the  lamination  of  shaly  strata. 
The  foliation  and  bedding,  however,  are  commonly 
more  or  less  puckered  and  contorted. 

Now  all  rocks  are  traversed  by  natural  division- 
planes  or  joints,  and  these,  in  the  case  of  well-bedded 
strata,  are  usually  disposed  at  approximately  right 
angles  to  the  planes  of  bedding.  Thus,  as  we  have 
seen,  beds  of  sandstone,  etc.,  are  divided  up  into 
somewhat  quadrangular  or  cuboidal  blocks.  Old 
lava-flows,  in  like  manner,  often  show  at  least  two 
similar  sets  of  vertical  joints,  and  not  infrequently 
these  are  cut  by  a  third  set,  disposed  at  approxi- 
mately right  angles  to  the  others.  Not  a  few  bedded 
igneous  rocks  and  intrusive  "  sheets,"  again,  assume 
a  more  or  less  columnar  aspect,  owing  to  the  sym- 
metrical arrangement  of  the  joints.  In  amorphous 
masses  of  crystalline  rocks,  on  the  other  hand,  uni- 
form jointing  as  a  rule  is  absent.  Their  division- 
planes  run  in  various  directions,  and  are  often 
extremely  irregular.  In  some  places  they  may  be 
very  closely  set,  in  other  places  they  are  far  apart. 


198  EARTH  SCULPTURE 

Thus  while  bedded  strata  of  all  kinds,  breaking  up 
along  the  joints,  tend  to  give  rise  to  rectangular  feat- 
ures at  the  surface,  amorphous  crystalline  rocks, 
quarried  by  epigene  action,  generally  yield  irregular 
contours.  And  the  same  is  the  case  with  the  crystal- 
line schists,  the  jointing  of  which  is  as  a  rule  capri- 
cious and  uncertain. 

It  is  obvious,  therefore,  that  surface-features  must 
be  greatly  influenced  by  the  character  of  rock-joints. 
Apart  altogether  from  other  geological  structures, 
joints  must  largely  determine  the  physiography  of 
the  surface.  To  such  an  extent  is  this  the  case,  that 
it  is  generally  easy  to  tell  at  a  glance  whether  any 
particular  mountain  is  composed  of  amorphous  crys- 
talline rocks,  of  schists,  or  of  regularly  bedded  strata. 
Mountains  carved  out  of  horizontal  strata  tend,  as  we 
have  seen,  to  assume  pyramidal  forms,  while  in  the 
case  of  inclined  beds  erosion  and  denudation  result 
in  the  formation  of  escarpments  and  dip-slopes.  This, 
however,  only  holds  true  when  relatively  hard  beds 
are  intercalated  among  a  series  of  softer  strata. 
Should  the  rocks  throughout  be  of  much  the  same 
consistency  no  escarpments  will  be  developed,  but  the 
whole  will  wear  away  equally,  and  so  give  rise  to  a 
gently  undulating  surface.  Usually,  however,  a  thick 
series  of  strata  will  be  found  to  comprise  rocks  of 
various  degrees  of  durability  ;  and  in  general,  there- 
fore, bedded  rocks,  whether  horizontal  or  inclined, 
tend  to  yield  rectangular  outlines.  But  when  the  dip 
greatly  increases,  and  the  strata  are  more  or  less  vio- 


INFLUENCE  OF  ROCK  CHARACTER  199 

lently  contorted,  the  beds  are  often  crushed  and  con- 
fusedly shattered  or  jointed,  while  at  the  same  time 
the  rocks  themselves  may  become  metamorphosed, 
and  eventually  pass  into  the  condition  of  schists. 
Rectangular  outlines  are  thus  gradually  replaced  by 
the  jagged,  rough,  and  abrupt  configuration  which  is 
so  characteristic  of  slaty  and  schistose  or  foliated 
rocks. 

Amongst  the  crystalline  schists  rectangular-  out- 
lines are  not  common.  Now  and  again,  however, 
when  different  kinds  of  schists  rapidly  alternate  in 
successive  sheets  or  beds,  some  will  almost  certainly 
weather  more  rapidly  than  others.  The  outcrops  of 
the  less  yielding  rocks  will  thus  tend  to  project ;  but 
as  jointing  is  usually  irregular  and  confused,  such  out- 
crops seldom  show  rectangular  outlines.  Exception- 
ally, well-marked  escarpments  may  be  met  with,  but 
the  general  high  dip  and  contorted  character  of  the 
rocks  forbid  such  formations.  When  steep  wall-like 
outcrops  of  schists  occur,  they  have  very  often  been 
determined  by  the  presence  of  normal  faults  or  of 
thrust-planes.  In  short,  while  the  foliation  and 
pseudo-bedding  of  schistose  rocks  now  and  again 
give  rise  to  surface-features  which  are  more  charac- 
teristic of  truly  bedded  strata,  yet  such  features  are 
apt  to  be  strongly  modified  by  the  vagaries  of  the 
jointing. 

In  amorphous  crystalline  masses,  which  show 
neither  bedding  nor  foliation,  the  character  of  the 
joints  usually  varies  with  the  nature  of  the  rock.  In 


200  EARTH  SCULPTURE 

granite,  for  example,  there  are  usually  three  sets  of 
joints,  one  of  which  traverses  the  rock  in  an  approxi- 
mately horizontal  direction,  or  may  have  a  dip  now 
in  one  direction,  now  in  another.  The  vertical  joints 
often  cut  each  other  at  right  angles,  but  not  infre- 
quently they  meet  at  more  or  less  acute  angles.  In 
addition  to  these  main  joints,  however,  there  are  often 
others.  Sometimes  the  joints  are  wide  apart,  and 
they  "then  enclose  large  rectangular  or  rhomboidal 
blocks.  At  other  times  they  are  set  so  closely 
together  that  the  rock  when  exposed  breaks  up  into 
a  mass  of  angular  debris.  As  the  character  of  the 
jointing  varies  in  this  way  within  narrow  limits,  the 
rock  tends  to  assume  broken  interrupted  contours. 
On  the  other  hand,  when  the  disposition  of  the  joint- 
planes  is  more  regular  and  better  defined,  the  hori- 
zontal joints  maintaining  their  direction  for  some 
distance,  granite  not  infrequently  breaks  up  as  if  it 
were  a  bedded  igneous  rock.  A  mountain-wall  so 
constructed  rises  in  a  series  of  gigantic  steps,  like 
tiers  of  cyclopean  masonry,  interrupted  by  entering 
and  re-entering  angles.  Where  the  "  horizontal " 
joints  are  much  inclined  a  corresponding  change  in 
the  direction  of  the  main  rock-ridges  and  reefs  may 
be  observed.  Not  infrequently,  however,  the  hori- 
zontal jointing  is  obscure  and  ill-defined  or  even 
wanting,  and  the  chief  contours  of  the  surface  are 
then  determined  by  the  vertical  joints  alone.  Under 
such  conditions  the  mountain-slope  shows  irregular 
vertical  or  steeply  inclined  *  walls,  ridges,  and  but- 


Joints  in  granite,  Glen  Eunach,  Cairngorm. 


INFLUENCE  OF  ROCK  CHARACTER  201 

tresses,  which  often  run  into  each  other  as  they  are 
followed  upwards,  and  may  eventually  taper  off  to  a 
point.  (See  Plate  I.) 

The  influence  of  joints,  however,  is  apt  to  be 
greatly  obscured  by  the  manner  in  which  rocks  them- 
selves disintegrate  and  crumble  down.  The  sharply 
angular  rock-faces  defined  by  joints  are  slowly  or 
more  rapidly  eaten  into  by  epigene  action,  and  the 
rock  exfoliates  or  crumbles  down  irregularly  accord- 
ing to  its  character.  Indeed,  this  rotting  action  has 
often  proceeded  very  far  before  the  joint-faces  are  laid 
bare.  When  a  mass  of  rock,  losing  its  support,  falls 
away,  the  new  surface  exposed  has  already  become  to 
a  larger  or  smaller  extent  disintegrated  and  decom- 
posed, so  that  frost  and  rain  are  enabled  rapidly  to 
reduce  and  modify  it.  Hence  the  sharp  irregular 
outlines  which  joints  naturally  tend  to  produce  are, 
in  the  case  of  such  rocks  as  granite,  generally  rounded 
off.  Basalt-rocks  in  like  manner  often  weather  readily 
and  become  decomposed  and  disintegrated  along 
planes  of  jointing,  and  thus  give  rise  to  a  somewhat 
rounded  and  lumpy  configuration.  But  there  is  often 
much  diversity  of  surface  displayed  by  one  and  the 
same  rock-mass,  the  basalt  in  some  places  weathering 
rapidly  into  rounded  forms,  while  in  other  places, 
especially  where  the  rock  is  fine-grained  and  compact, 
the  sharp  angles  of  the  jointing  are  better  preserved. 
(See  Plate  II.) 

The  usually  finer-grained  rhyolites,  trachytes,  ande- 
sites,  and  phonolites  are  not  as  a  rule  so  readily  dis- 


202  EARTH  SCULPTURE 

integrated  as  normal  granites  and  basalts.  Their 
joints,  moreover,  are  not  only  less  uniform,  but  fre- 
quently very  abundant  and  closely  set.  Such  rocks, 
therefore,  are  readily  broken  up.  Mountains  carved 
out  of  them  usually  show  sharp  crests  and  peaks, 
while  their  slopes  are  hidden  under  curtains  of  angular 
debris,  through  which  ever  and  anon  are  protruded 
reefs,  ridges,  buttresses,  and  bastions  of  such  portions 
of  the  rock-mass  as  are  less  profusely  jointed.  (See 
Fig.  78,  p.  203.) 

In  short,  we  may  say  that  every  well-marked  rock- 
type  breaks  up  and  weathers  in  its  own  way,  so  that 
under  the   influence  of  denudation   each   assumes  a 
particular  character.     We  see  this  even  in  the  case  of 
well-bedded  aqueous  rocks.      Planes  of  bedding  and 
jointing  no  doubt  are  the  lines   of  weakness   along 
which  rocks  most  readily   yield,  but  each  individual 
rock-species  weathers  after  its  own  fashion — the  dif- 
ferent kinds  of  shale,  sandstone,   conglomerate,   and 
limestone  are  decomposed,  disintegrated,  and  crum- 
bled down  at  different  rates,  and   each  in  a  special 
way,  according  to  its  mineralogical  composition  and 
state  of  aggregation.     Thus,  although  a  region  built 
up    of    bedded  aqueous    rocks   may   show  the   same 
general  configuration    throughout — horizontal  strata 
giving  rise  to  pyramidal-shaped  hills  and  mountains, 
while  inclined  strata  of  variable  consistency  present 
us  usually  with  a  series  of  escarpments  and  dip-slopes 
—yet  with  all  this  sameness  the  details  of  rock-sculpt- 
uring may  be  singularly  varied.       And  the  same  is 


PLATE 


Weathering  of  joints  in  granite,  Cairngorm  Mountains. 


204  EARTH  SCULPTURE 

true  of  the  crystalline  schists.  Mountains  composed 
of  such  rocks  have  much  the  same  general  configura- 
tion. But  when  viewed  in  detail  they  show  with 
every  change  in  the  character  of  the  rock  some  corre- 
sponding change  in  the  aspect  of  the  surface.  Again, 
in  the  case  of  granite,  gabbro,  and  other  massive 
igneous  rocks,  all  these  doubtless  break  up  and  pro- 
duce characteristic  configurations.  But  in  each  indi- 
vidual case  we  may  note  many  details  of  sculpturing 
which  are  not  the  result  of  jointing,  but  of  variations 
in  the  texture,  and  even  in  the  mineralogical  compo- 
sition of  the  rock.  We  may  note  further  that  one 
and  the  same  kind  of  rock  does  not  necessarily  always 
present  quite  the  same  aspect  under  weathering  and 
erosion.  Much  will  depend  on  the  character  of  the 
climate,  on  the  elevation  of  the  region  in  which  it 
occurs,  and  on  the  nature  of  the  surface,  whether,  for 
example,  that  be  steeply  or  gently  inclined. 

The  characteristic  forms  assumed  by  rocks  are,  of 
course,  best  seen  in  places  where  these  are  well  ex- 
posed. In  low-lying  tracts  the  rock-surface  is  usually 
more  or  less  concealed  beneath  alluvial  deposits  and 
other  superficial  accumulations  of  epigene  action.  It 
is  in  river-ravines  and  along  the  sea-coast,  or  better 
still  amongst  the  mountains,  that  rock-weathering 
must  be  studied.  Even  at  the  higher  levels,  however, 
the  rocks  are  often  largely  concealed  under  their  own 
ruins.  Sheets  and  cones  of  debris  extend  downwards 
from  the  base  of  every  projecting  cliff  and  buttress. 
Hence  in  the  case  of  mountains  carved  out  of  bedded 


INFLUENCE  OF  ROCK  CHARACTER  205 

rocks,  the  rectangular  outlines  tend  to  become  ob- 
scured, projecting  rock-ledges  gradually  disappear 
under  piles  of  ctibris,  and  a  smooth  slope  may  replace 
in  whole  or  in  part  the  rectangular  corbel-steps  of  the 
typical  pyramid,  while  steep  escarpments  may  be 
smoothed  off  to  more  or  less  gentle  inclines.  In  the 
case  of  mountains  composed  of  schistose  rocks  the 
general  steep  inclination  and  contorted  character  of 
the  bedding  and  the  varied  character  of  the  rocks 
themselves  favour  the  preservation  of  abrupt  and 
irregular  slopes.  There  is  a  general  absence  of 
horizontal  or  gently  inclined  platforms  upon  which 
debris  may  come  to  rest.  The  great  mass  of  the 
material  loosened  and  detached  by  weathering  rolls 
and  shoots  downwards  to  the  screes  accumulating  at 
the  base  of  the  mountains.  These,  as  denudation 
advances,  are  of  course  continually  extending  up- 
wards. But  the  characteristic  configuration  of  the 
rocks  above  the  scree-line  is  maintained,  and  not 
obscured,  as  so  frequently  happens  in  the  case  of 
horizontal  or  gently  inclined  strata.  Amorphous 
igneous  masses  break  up  in  so  diverse  a  manner,  that 
mountains  composed  of  such  often  show  much  variety 
of  feature.  The  upper  limits  of  the  scree-line  are 
very  tortuous,  here  sweeping  up  almost  to  the  very 
crest  of  a  mountain,  there  hugging  the  base  of  gaunt 
cliffs  and  precipices.  Or,  when  horizontal  jointing  is 
well  defined,  we  may  have  a  succession  of  abrupt  ledges 
breaking  the  continuity  of  a  scree-slope.  When,  on  the 
other  hand,  vertical  joints  are  most  pronounced  bare 


206  EARTH  SCULPTURE 

rock-walls  and  steep  ridges  rise  more  or  less  abruptly 
above  the  limits  of  the  depressed  scree-line  below. 

In  regions  subject  to  well-marked  dry  and  rainy 
periods  even  low  grounds  and  plateaux  not  infre- 
quently show  much  bare  rock.  This  is  due  to  the 
fact  that  disintegrated  rock-material  tends  to  be 
swept  rapidly  downwards  by  heavy  torrential  rains. 
Should  the  land  be  well  clothed  with  vegetation,  the 
reduction  of  the  surface  is  much  retarded.  The  rocks 
may  become  rotted  to  great  depths,  as  in  Brazil,  but 
the  decomposed  material  remains  in  situ.  Where  veg- 
etable life  in  such  latitudes  is  less  prolific  the  surface 
becomes  scorched  and  dried,  and  disintegrated  rock- 
material  is  readily  removed  when  the  rainy  season 
comes  round.  Under  these  conditions  the  surface- 
features,  due  to  epigene  action,  are  usually  strongly 
pronounced.  A  plateau  of  granitoid  rock,  for  ex- 
ample, owing  to  inequalities  of  structure,  texture, 
and  composition,  often  yields  a  highly  diversified  sur- 
face ;  rounded  blocks  and  boulders  of  all  shapes  and 
sizes  appear  scattered  broadcast,  while  sporadic 
masses,  stacks,  cones,  tors,  crags,  and  peaks,  and  ir- 
regular winding  gullies  and  depressions,  are  every- 
where encountered.  But  the  same  phenomena,  if 
somewhat  less  prominently  developed,  are  seen  again 
and  again  in  temperate  latitudes.  The  "  tors  "  of 
Cornwall  are  in  their  way  as  striking  as  the  kopjes 
of  Mashonaland.  Many  other  kinds  of  rock,  after 
long  exposure  to  the  weather,  present  similar  fantastic 
outlines.  The  "  Quadersandstein  "  of  Saxon  Switzer- 


INFLUENCE  OF  ROCK  CHARACTER  207 

land,  for  example,  which  over  considerable  areas  lies 
in  approximately  horizontal  strata,  has  suffered  great 
erosion,  the  characteristic  features  of  the  region  being 
conical  hills  or  pyramids  and  broad  bastions,  along 
the  flanks  of  which  the  naked  rock  appears.  Thus 
exposed  to  weathering,  the  sandstones  yield  along  the 
vertical  joint-planes  and  fall  away  somewhat  unequally, 
and  so  stacks  and  columns  eventually  become  separ- 
ated from  the  main  rock-masses,  and  often  weather 
into  odd  and  picturesque  forms. 

The  surface-features  assumed  by  limestone  are  very 
characteristic,  and  these,  as  in  the  case  of  all  stratified 
rocks,  are  determined  by  bedding  and  jointing.  But 
the  soluble  character  of  limestone  causes  it  to  weather 
in  a  manner  peculiar  to  itself.  Bare  surfaces  are  eaten 
into,  and  become  irregularly  honeycombed  and  fur- 
rowed— the  rock,  in  short,  is  corroded  by  the  chemical 
action  of  rain.  Should  the  ground  be  steeply  inclined, 
the  surface  of  the  limestone  shows  numerous  more  or 
less  parallel  gutters  and  trenches,  separated  by  narrow 
ridges  which  are  frequently  sharp  and  knife-edged. 
Upon  gentler  slopes  the  gutters  are  less  regular,  and 
the  ridges  are  often  somewhat  rounded  ;  the  whole  sur- 
face, indeed,  may  be  rudely  mammillated,  and  tra- 
versed or  interrupted  by  abrupt  furrows  and  smoother 
depressions.  These  appearances  are  most  marked 
when  the  limestone  is  pure  ;  when  it  contains  much 
insoluble  matter  the  characteristic  ridges  and  trenches, 
rounded  humps  and  hollows,  are  seldom  well  devel- 
oped. It  is  needless  to  add  that  endless  modifications 


208  EAR  TH  SCULP  TURE 

of  the  surface-forms  referred  to  result  from  the  char- 
acter of  the  bedding  and  jointing,  the  latter  having 
often  determined  the  direction  of  the  guttejs  and  fur- 
rows. The  appearances  now  described  (the  Kar- 
renfelder  of  German  writers)  are  not  confined  to 
any  particular  level,  but  occur  at  all  levels,  being 
most  pronounced,  however,  on  high  plateaux  and  in 
mountain-regions  where  there  is  little  or  no  vegetable 
covering.  Excellent  examples  are  met  with  in  the 
calcareous  tracts  of  the  Alps,  in  the  Jura,  in  the  plat- 
eaux of  the  Cevennes,  in  the  Pyrenees,  at  Gibraltar, 
and  many  other  places  in  Europe. 

Owing  to  its  solubility,  limestone  is  not  only  cor- 
roded at  the  actual  surface,  but  joints  and  fissures  are 
widened  by  the  same  solvent  action,  and  thus,  in  time, 
underground  channels  are  licked  out,  and  streams 
and  rivers  are  gradually  conducted  into  subterranean 
courses.  These  now  become  widened  and  deepened, 
not  only  by  chemical  solution,  but  by  the  mechanical 
action  of  running  water.  Thus,  in  limestone  regions, 
the  whole  drainage  may  be  directed  underground. 
Considerable  streams  and  rivers  plunge  suddenly  into 
the  depths,  and  after  a  longer  or  shorter  course  may 
reappear  at  the  surface,  or  they  may  flow  on  until 
they  make  their  final  escape  on  the  floor  of  the  sea. 
The  surface  of  a  limestone  country  is  often  drilled  by 
more  or  less  vertical  holes  and  pipes  of  variable  width, 
which  communicate  directly  with  subterranean  streams 
and  rivers.  These  pipes  are,  no  doubt,  in  many  cases, 
licked  out  by  meteoric  water,  but  not  infrequently 


INFLUENCE  OF  ROCK  CHARACTER  209 

they  are  caused  by  the  collapse  of  the  undermined 
rocks.  Owing  to  various  causes,  engulfed  streams 
now  and  again  abandon  their  courses,  and  work  their 
way  to  lower  levels,  and  in  course  of  time  such  aban- 
doned channels  may  become  disclosed  by  the  falling- 
in  of  the  roof,  or  by  the  more  gradual  denudation  and 
truncation  of  the  rock  by  surface-action.  Hence,  in 
regions  built  up  of  calcareous  rocks,  caves  are  of  com- 
mon occurrence,  many  of  them  being  of  large  dimen- 
sions, and  often  branching  in  all  directions. 

Caves  and  other  hollows  are  not  infrequently  worked 
out  by  weathering  in  many  other  kinds  of  rocks,  but  in 
no  case  do  they  attain  the  size  of  those  which  we  so 
commonly  encounter  in  areas  occupied  by  limestone, 
as  will  be  shown  in  a  succeeding  chapter. 

We  need  not,  however,  enter  into  further  detail  as 
regards  the  characteristic  weathering  of  particular 
rocks.  It  is  enough  for  our  purpose  to  recognise  the 
fact  that  composition  and  texture  play  no  unimport- 
ant part  in  determining  the  aspect  assumed  by  rocks 
under  denudation.  In  preceding  pages  we  have  dis- 
cussed the  origin  of  the  salient  features  of  a  land- 
surface.  Looked  at  broadly,  it  is  obvious  that  the 
more  elevated  and  more  depressed  areas  owe  their 
existence  primarily  to  movements  of  the  earth's  crust. 
Thus  all  the  great  mountain-tracts  and  plateaux  of 
Europe  may  be  looked  upon  as  regions  of  relative 
uplift,  while  the  broad  low  grounds  above  which  they 
rise  may  be  described  in  general  terms  as  regions  of 
relative  depression.  In  a  word,  the  larger  features  of 


2io  EARTH  SCULPTURE 

the  land  have  been  blocked  out  by  subterranean 
action,  they  are  the  result  of  crustal  deformation. 
Viewed  from  a  nearer  standpoint,  however,  we  recog- 
nise that  every  feature  due  to  deformation  has  been 
more  or  less  profoundly  modified  by  denudation, 
guided  and  determined  by  the  geological  structure 
and  relative  durability  of  the  rocks.  Approaching 
still  nearer,  we  see  how  each  particular  kind  of  rock 
wears  away  in  some  particular  and  characteristic  fash- 
ion, so  that  surface-features  vary  infinitely  in  detail 
quite  independently  of  the  geological  structure.  Thus 
the  part  played  by  subterranean  action  is  merely  to 
provide  the  rough  block  which  the  epigene  agents 
subsequently  sculpture  into  shape.  With  few  excep- 
tions, the  land-features  that  now  meet  our  eye  are  the 
direct  result  of  erosion  and  accumulation,  the  modify- 
ing influence  of  which  is  always  more  or  less  conspic- 
uous even  in  cases  of  recent  crustal  deformation. 

Now  if  it  be  true  that  the  character  of  a  land-sur- 
face is  determined  by  geological  structure  and  the 
nature  of  the  rocks,  we  should  expect  to  meet  with 
very  considerable  diversity  of  configuration  in  regions 
built  up  of  many  varieties  of  rock  arranged  in  many 
different  ways.  And  such  undoubtedly  is  the  case  ; 
but  it  is,  less  true  of  temperate  and  northern  regions 
than  of  more  southerly  latitudes.  Not  that  the  influ- 
ence of  rock-structure  is  ever  quite  lost  even  in  the 
former,  but  it  is  often  obscured.  In  the  contours 
of  the  higher  Alps,  for  example,  it  is  conspicuous 
enough,  but  the  lower  mountain-slopes  not  infre- 


INFLUENCE  OF  ROCK  CHARACTER     211 

quently  fail  to  show  it,  or  show  it  much  less  plainly. 
Further  north,  as  in  our  country  and  in  Scandinavia, 
undulating  and  flowing  configurations  prevail  amongst 
the  mountains.  Broken  and  serrated  outlines  are  sel- 
dom seen,  and  usually  only  at  the  higher  elevations. 
Mountains  built  up  of  bedded  rocks,  of  schists,  of  mas- 
sive igneous  rocks,  are  not  so  strongly  differentiated 
as  similar  mountain-masses  are  in  more  southern 
lands.  It  is  only  when  they  are  looked  at  more  closely 
that  the  influence  of  geological  structure  and  petro- 
graphical  character  becomes  apparent.  Everywhere, 
however,  we  find  that  this  influence  has  been  more 
or  less  interfered  with  ;  mountains  which,  under  the 
ordinary  action  of  the  atmosphere,  must  have  assumed 
serrated  crests  and  peaks,  appear  instead  with  rounded, 
smoothed,  and  softened  outlines;  projecting  buttresses, 
reefs,  and  ridges  have  lost  much  of  their  angularity, 
and  escarpments  likewise  are  frequently  bevelled  off. 
These  remarkable  modifications  of  the  surface  are 
due  to  glaciation.  There  is  no  reason  to  doubt  that 
before  the  advent  of  the  Ice  Age  rock  character  and 
geological  structure  were  as  strongly  expressed  in 
the  configuration  of  our  hills  and  valleys  as  they  are 
now  in  regions  which  have  never  experienced  glacia- 
tion. Indeed,  so  long  a  time  has  elapsed  since  the 
disappearance  of  our  ice-fields  and  glaciers,  that  the 
smoothed  and  rounded  surfaces  are  again  breaking 
up,  and  the  more  irregular  and  angular  contours  and 
outlines  which  obtained  in  preglacial  ages  are  thus  in 
process  of  gradual  restoration. 


CHAPTER  X 

LAND-FORMS  MODIFIED  BY  GLACIAL  ACTION 

GEOLOGICAL    ACTION   OF  EXISTING   GLACIERS — EVIDENCE   OF    ERO- 
SION  ORIGIN  OF    THE   GROUND-MORAINE  :    ITS    INDEPENDENCE 

OF  SURFACE-MORAINES — INFRAGLACIAL  SMOOTHING  AND  POL- 
ISHING, CRUSHING,  SHATTERING,  AND  PLUCKING — GEOLOGI- 
CAL ACTION  OF  PREHISTORIC  GLACIERS — GENERAL  EVIDENCE 
SUPPLIED  BY  ANCIENT  GLACIERS  OF  THE  ALPS. 

AT  the  close  of  the  last  chapter  reference  was  made 
to  the  fact  that  the  surface-features  of  certain 
regions  have  been  modified  by  subsequent  glacial 
action.  This  action,  as  we  have  indicated,  tends  to 
efface  or  obscure  the  characteristic  forms  assumed  by 
rock-masses  under  the  influence  of  weathering.  In 
other  words,  ice  is  an  eroding  agent,  but  it  works  in 
a  different  way  from  the  ordinary  epigene  agents. 
While  the  latter  tend  to  produce  manifold  irregulari- 
ties of  the  surface,  and  to  develop  angular  outlines 
for  the  most  part,  the  former  tends,  on  the  other  hand, 
to  smooth  away  inequalities  and  to  replace  angular 
outlines  with  rounded  contours.  It  is  demonstrable, 
therefore,  that  ice  is  an  eroding  agent,  but  some  geo- 
logists have  doubted  whether  it  is  very  effective,  and 
are  of  the  opinion  that  the  utmost  it  can  do  is  to 


GLACIAL  ACTION  213 

smooth  and  abrade  to  a  very  limited  extent.  As  it  is 
important,  from  our  present  point  of  view,  that  we 
should  clearly  understand  this  question  of  glacial 
erosion,  we  may  consider  the  evidence  in  some  little 
detail. 

For  this  purpose  we  may  approach  the  subject  much 
in  the  same  way  as  a  geologist  would  do  were  he  en- 
deavouring to  prove  for  the  first  time  that  rivers  are 
potent  agents  of  erosion.  Doubtless,  in  such  a  case, 
his  first  care  would  be  to  describe  the  work  done  by 
existing  rivers  ;  thereafter  he  would  depict  the  char- 
acter and  attempt  to  set  forth  the  precise  origin  of 
alluvial  terraces,  plains,  and  deltas  ;  and,  finally,  he 
would  adduce  evidence  to  prove  that  all  such  forma- 
tions are  products  of  erosion,  and  that  by  the  gradual 
removal  of  such  products  valleys  have  been  originated 
or  deepened.  In  like  manner  we  shall  consider  first 
the  character  of  existing  glacial  action  ;  then  we  shall 
inquire  into  the  nature  and  origin  of  ancient  glacial 
accumulations ;  and  finally  we  shall  show  how  these 
last  are  evidence  of  extensive  glacial  erosion,  and 
how,  by  their  removal,  valleys  have  been  widened 
and  deepened,  and  rock-basins  of  particular  kinds 
have  been  formed. 

i.  The  geological  action  of  existing  glaciers. — The 
most  obvious  work  performed  by  an  Alpine  glacier 
is  that  of  transport  and  accumulation.  The  wreck  of 
the  adjacent  mountains,  strewn  upon  its  surface,  is 
continually  carried  forward,  and  eventually  heaped 
up  in  the  form  of  terminal  moraines.  The  infragla- 


214  EARTH  SCULPTURE 

cial  debris  extruded  at  the  lower  end  of  the  ice-flow 
bears,  usually,  a  very  small  proportion  to  the  supply 
of  rock-rubbish  travelling  at  the  surface.  This,  how- 
ever, is  not  invariably  the  case,  even  in  the  Alps. 
Not  infrequently  small  "  summit  glaciers,"  lying  upon 
mountain-slopes,  bear  no  superficial  detritus,  while 
infraglacial  debris,  nevertheless,  is  constantly  being 
extruded  at  their  lower  ends.  Thus  the  small  Stampfl- 
kees  Glacier  (Zillerthal),  overlooked  by  hardly  any 
exposed  rock-surfaces,  and  consequently  carrying  lit- 
tle or  no  superficial  rock-rubbish,  yet  exhibits  at  its 
terminal  front  a  bottom-  or  ground-moraine  some  ten 
or  fifteen  feet  thick.  But  that  which  is  the  exception 
in  Alpine  lands  is  the  rule  in  Arctic  regions.  The 
tongues  of  ice  protruding  from  the  vast  mer  de  glace 
of  Greenland  are  almost  entirely  free  from  the  super- 
ficial debris,  and  yet  they  eject  ground-moraine  in 
abundance.  The  same,  as  we  shall  see  presently,  is 
the  case  with  most  of  the  Norwegian  glaciers.  It  is 
obvious,  therefore,  that  the  relative  importance  of 
ground-moraine,  as  a  product  of  glacial  action,  is  really 
greater  than  a  glance  at  the  phenomena  of  any  ordin- 
ary Alpine  glacier  would  at  first  lead  one  to  suppose. 
The  general  nature  of  Alpine  ground-moraine  is 
well  known.  It  consists  simply  of  an  aggregate  of 
rock-fragments,  grit,  sand,  and  mud  or  clay,  often 
frozen  or  pressed  together,  and  so  included  in  the 
lower  or  basal  portion  of  the  glacier.  Many  of  the 
stones  are  subangular  and  blunted,  and  striated, 
smoothed,  or  polished  on  one  or  more  sides.  No 


GLACIAL  ACTION  215 

one  doubts  that  this  material  has  travelled  under- 
neath, and  partly  enclosed  in  the  ice-flow,  and  that 
the  rock-surface  over  which  it  has  been  carried  is 
abraded,  smoothed,  and  polished  by  its  filing  action. 
Everyone,  in  short,  admits  that  some  degree  of  ero- 
sion is  the  result  of  glacial  action.  Were  that  action 
entirely  confined  to  mere  abrasion  and  smoothing  of 
rock-surfaces,  it  yet  could  hardly  be  considered  insig- 
nificant. The  fine  powder  or  flour  of  rock  which 
renders  all  glacial  rivers  turbid,  shows  that  glacial 
grinding  is  really  of  great  importance.  It  has  been 
computed,  for  example,  that  the  river  extending  from 
Aar  Glacier  carries  away  daily  280  tons  of  solid  mat- 
ter in  suspension.  Again,  the  Justedal  Glacier,  drain- 
ing an  ice-field  820  square  miles  in  extent,  discharges 
in  a  summer  day  1968  tons  of  sediment.  This  is  in 
excess  of  the  average  daily  discharge  during  the  year, 
which  Helland  estimates  at  180  million  kilogrammes. 
To  this  should  be  added  the  mineral  matter  carried  in 
solution,  amounting  to  13  million  kilogrammes,  so 
that  solid  and  dissolved  materials  taken  together  come 
up  to  189,950  tons.  This  would  form  a  mass  equal 
to  90,252  cubic  yards.  According  to  the  same  geolo- 
gist, the  Vatnajokull  (Iceland),  draining  an  ice-field 
ten  times  larger  than  that  of  the  Justedal,  discharges 
annually  14,763,000  tons  of  sediment — an  amount 
equal  to  7, 194,000  cubic  yards  of  rock.  Thus,  even 
if  a  glacier  does  no  more  than  abrade  and  smooth  its 
bed,  the  amount  of  rock  ground  into  powder  is  neither 
insignificant  nor  unimportant. 


216  EARTH  SCULPTURE 

But  is  this  all  the  erosion  that  a  glacier  accom- 
plishes ?  What  about  the  debris  of  its  ground-moraine 
— whence  is  that  derived  ?  Professor  Heim  and 
others  maintain  that  in  the  case  of  a  large  number  of 
glaciers  (Alps,  Himalaya,  New  Zealand)  infraglacial 
detritus  comes  chiefly  from  superficial  sources.  Over- 
lying morainic  rubbish,  it  is  supposed,  finds  its  way 
through  crevasses  to  the  bottom  of  the  ice.  Now 
there  can  be  no  doubt  that  surface-moraines  are 
frequently  engulfed  in  crevasses  ;  but  then  the  rock- 
rubbish  engulfed  in  this  way  sooner  or  later  reap- 
pears at  the  surface  of  the  glacier  further  down  the 
valley.  Obviously  in  such  cases  the  debris  does  not 
descend  to  the  bottom  of  the  glacier,  but  is  simply 
engorged  at  some  distance  from  the  surface,  and  again 
becomes  exposed,  owing  to  the  curving  upwards  of 
the  lines  or  planes  of  flow  and  the  ablation  of  the 
surface.  If  crevasses  penetrated  the  whole  thickness 
of  a  glacier,  doubtless  debris  plunging  into  them 
might  well  reach  the  bottom  of  the  ice,  and  be  in- 
cluded as  ground-moraine.  But  the  plasticity  of  ice 
necessarily  limits  the  depths  to  which  a  crevasse  can 
extend.  The  larger  glaciers,  according  to  Heim,  are 
never  penetrated  to  the  bottom  by  crevasses,  which 
when  not  kept  open  and  deepened  by  ablation  do  not 
exceed  a  depth  of  100-150  metres.  Superficially 
carried  rock-rubbish,  therefore,  can  reach  the  bottom 
of  a  moderately  thick  glacier  only  along  the  margin, 
where  the  crevasses  open  to  the  rock-head.  Here 
and  there,  perhaps,  debris  may  occasionally  descend 


GLACIAL  ACTION  217 

by  moulins  ;  but  as  a  rule  the  bed  of  such  a  glacier 
can  receive  only  a  very  meagre  supply  of  rock-frag- 
ments from  above.  And  if  this  be  the  case  with  the 
relatively  small  glaciers  of  the  Alps,  it  must  be  the 
same  in  a  more  marked  degree  with  those  of  high 
northern  and  Arctic  lands. 

Reference  has  already  been  made  to  the  fact  that 
even  in  the  Alps  certain  summit-glaciers  are  so  placed 
that  no  debris  is  showered  upon  them,  and  yet  these 
glaciers  extrude  more  or  less  conspicuous  ground- 
moraines.  In  a  word,  the  existence  of  ground- 
moraines  does  not  depend  upon  the  presence  of 
superficial  moraines.  The  latter  are  not  infrequently 
wanting ;  the  former,  on  the  contrary,  never  are. 
This  is  well  seen  in  the  case  of  the  Norwegian  gla- 
ciers, which,  as  compared  with  those  of  the  Alps, 
might  be  described  as  almost  devoid  of  surface-dS$rtf. 
Nevertheless,  ground-moraines  are  always  in  evidence, 
appearing  not  only  under  the  tongue-like  glaciers 
which  protrude  from  the  plateau  ice-fields,  but  at  the 
base  of  the  more  or  less  steep  walls  in  which  those 
ice-fields  usually  terminate. 

The  great  development  of  superficial  moraines  in 
the  Alps  as  contrasted  with  their  meagre  appearance 
in  Scandinavia  is  easily  explained.  In  the  former 
region  we  have  a  complicated  series  of  mountain- 
groups  and  chains,  the  crests  of  which  overlook  pro- 
found cirque-like  depressions.  It  is  in  these  broad 
and  deep  troughs  and  basins  that  snows  accumulate 
to  form  the  reservoirs  from  which  glaciers  flow. 


2 1 8  EARTH  SCULP TURE 

Even  at  its  very  source,  therefore,  an  Alpine  glacier 
has  rock-de&rt's  supplied  to  it  from  above,  and  as  it 
passes  down  its  mountain-valley  frost  and  avalanches 
keep  up  a  constant  bombardment,  so  that  the  farther 
it  flows  the  greater  becomes  the  amount  of  detritus 
eventually  piled  up  in  its  terminal  moraines.  Nor- 
way, on  the  other  hand,  is  a  lofty  plateau,  more  or 
less  deeply  trenched  by  fiords  and  valleys.  The 
snows,  therefore,  accumulate  upon  a  wide  and  rela- 
tively flat  or  undulating  surface,  not  dominated  by 
peaks  or  ridges.  In  the  central  part  of  a  Norwegian 
snow-field  the  surface  is  more  or  less  continuous,  and 
seldom  interrupted  by  crevasses.  Now  and  again, 
however,  these  are  encountered,  and  their  walls  show 
stratified  ntvt  above  graduating  downwards  into  com- 
pact ice.  Towards  the  margin  of  such  an  ice-field 
crevasses  become  more  frequent,  and  in  these  snow 
and  ntvt  are  seen  gradually  thinning-off  as  the  term- 
inal wall  is  approached,  until  at  last  the  blue  ice  is 
wholly  exposed.  In  short,  the  Scandinavian  plateau 
supports  true  ice-sheets,  comparable  in  all  respects, 
save  as  regards  their  extent,  to  the  great  "  inland  ice  " 
of  Greenland.  In  places,  longer  or  shorter  tongues 
of  ice  project  from  the  ice-sheet  into  valleys  ;  in  other 
places,  where  no  valleys  are  present,  the  sheet  simply 
terminates  in  a  continuous  ice-wall. 

Such  being  the  character  of  the  Scandinavian  ice- 
fields, we  need  not  wonder  at  the  absence  of  superfi- 
cial moraines.  No  mountains  overlook  the  plateaux  ; 
it  is  only  when  the  ice  creeps  outwards  into  valleys 


GLACIAL  ACTION  219 

that  it  is  liable  to  have  rock-rubbish  dumped  on  its 
surface.  Moreover,  the  course  of  such  valley-glaciers 
is  so  short  as  a  rule,  and  their  rate  of  flow  so  com- 
paratively rapid,  that  conspicuous  lateral  moraines 
cannot  be  accumulated.  It  is  further  noteworthy 
that  Norwegian  glaciers  do  not  form  prominent  ter- 
minal moraines,  and  these  are  composed  chiefly  of 
water-worn  gravel  and  blunted  and  subangular  stones. 
Sharply  angular  blocks  and  fragments  do  not  predom- 
inate as  in  the  end-moraines  of  Alpine  glaciers.  In 
a  word,  the  Norwegian  terminal  moraines  appear  to 
consist  mainly  of  infraglacial  and  fluvio-glacial  detritus, 
which  the  ice  builds  up  into  low  mounds  and  ridges. 
But  if  superficial  moraines  are  sparingly  devel- 
oped, the  same  is  not  the  case  with  ground-moraines. 
These  are  seen  not  only  under  the  glacier-tongues  in 
valleys,  but  they  are  conspicuous  likewise  under  the 
bordering  ice-walls  of  the  plateau  -  sheets.  Every- 
where, also,  from  the  margin  of  these  sheets,  as  from 
the  valley-glaciers,  flow  streams  and  torrents  of  turbid 
water. 

The  phenomena  exhibited  by  the  Scandinavian  ice- 
fields are  exemplified  on  a  much  larger  scale  in  Green- 
land. There,  as  in  Norway,  superficial  moraines  are 
entirely  wanting,  except  where  the  ice-sheet  protrudes 
long  tongues  into  mountain-valleys  and  fiords.  Where 
the  ice-sheet  terminates  upon  land  ground-moraines 
are  conspicuous.  Nansen,  for  example,  tells  us  that 
at  Austmannatjern,  where  he  left  the  inland  ice  after 
his  famous  traverse,  enormous  accumulations  of  mo- 


220  EARTH  SCULPTURE 

raine  were  seen.  These  were  of  true  infraglacial  ori- 
gin, consisting  largely  of  blunted  and  striated  stones, 
which  could  only  have  been  transported  by  the  ice  as 
ground-moraine.  No  Nunatakkr  occurred  within 
the  mer  de  glace  near  this  place,  and  not  a  vestige  of 
surface-moraine  was  visible.  Dr.  Hoist,  Dr.  Dry- 
galski,  and  others  have  referred  to  the  appearance  of 
ground-moraines  in  Greenland,  and  the  phenomena 
in  question  have  also  been  described  by  Professor 
Chamberlin.  The  latter  shows  that  the  tongues  of 
ice  proceeding  from  the  local  ice-caps  and  from  the 
great  inland  ice  are  crowded  towards  their  base  with 
ground-moraine,  the  lower  strata  of  the  ice  for  a 
thickness  of  50  to  70  feet  above  the  bottom  showing 
layers  and  irregular  sheets  of  clay,  mud,  sand,  stones, 
and  boulders,  all  of  which  are  of  infraglacial  origin, 
while  the  upper  and  much  thicker  mass  of  ice  is  free 
from  such  inclusions.  It  is  not  necessary  to  enter 
into  greater  detail,  but  it  may  be  added  that  in 
Greenland  as  in  Norway  turbid  water  escapes  in  large 
volume  from  the  "  inland  ice." 

Reflecting  upon  the  facts  thus  briefly  recapitulated, 
we  must  conclude  that  glaciers  are  powerful  agents  of 
erosion.  Not  only  do  they  grind,  smooth,  and  polish 
rock-surfaces,  as  everyone  admits,  but  they  also  quarry 
their  beds.  The  stones  and  boulders  of  the  ground- 
moraines  are  derived  directly  from  below  by  the  ice 
itself.  In  the  case  of  Alpine  glaciers,  no  doubt  debris 
may  occasionally  be  introduced  to  the  ground-moraine 
from  above  ;  but  this  descent  of  superficial  detritus 


GLACIAL  ACTION  221 

cannot  take  place  in  the  plateau-sheets  of  Scandi- 
navia, nor  in  the  local  ice-caps  of  the  great  "  inland 
ice  "  of  Greenland.  In  some  way  or  other,  rocks  under- 
lying a  glacier  are  liable  to  disruption  and  displace- 
ment ;  and  such,  we  cannot  doubt,  is  the  chief  source 
of  the  stones  and  grit  and  clay  of  ground-moraines 
generally.  There  is  direct  evidence,  indeed,  to  show 
that  glaciers  not  only  abrade  and  smooth,  but  rupture 
the  rocks  over  which  they  flow.  Professor  Heim 
refers  to  an  observation  of  Von  Escher  on  the  Zmutt 
Glacier,  underneath  which  were  seen  projecting  reefs 
of  schist  glaciated  atop,  which  had  been  fractured  and 
sundered  by  the  glacier.  Again,  Professor  Simony 
has  described  the  appearance  presented  on  the  bed  of 
one  of  the  Dachstein  Glaciers  (Karls-Eisfeld)  during 
the  temporary  retreat  of  the  ice.  What  struck  him 
most  was  not  so  much  the  smoothed  and  polished 
surfaces  as  the  broken  and  disrupted  masses,  the  shat- 
tering being  most  marked  in  places  where  the  rock- 
ledges  faced  the  direction  of  the  ice-flow.  The 
prevailing  character  of  the  erosion,  Professor  Simony 
remarks,  is  that  of  a  continuous  rock-shattering.  On 
the  north  side  of  the  glacier,  where  the  surface  had 
become  depressed  for  40  to  60  feet,  the  exposed  rocks 
showed  polishing  in  only  a  few  places,  glacial  pressure 
having  resulted  rather  in  a  wholesale  superficial  shat- 
tering, and  in  the  production  of  a  rubble  of  angular 
fragments. 

Similar  phenomena  have  been   observed  by  MM. 
Penck,  Bruckner,  and  Baltzer  at  the  Uebergossenen 


222  EARTH  SCULPTURE 

Aim.  During  the  past  thirty  years  this  glacier  has 
retreated  for  two  or  three  hundred  yards.  Its  de- 
serted bed  is  traversed  by  a  belt  of  hornblende- 
slate,  which,  like  the  adjacent  rock-masses,  is  well 
glaciated  and  sprinkled  with  large  striated  blocks  of 
gneiss.  In  some  places,  however,  the  hornblende- 
slate,  after  having  been  smoothed  and  polished,  has 
been  broken  up,  and  cUbris^  consisting  of  smaller  and 
larger  fragments  and  blocks,  polished  on  one  side  only, 
are  found  incorporated  in  ground-moraine  a  little  fur- 
ther down.  This  is  a  clear  case  of  infraglacial  quar- 
rying. Another  good  opportunity  of  studying  the 
results  of  modern  glacial  action  has  been  afforded  by 
the  retreat  of  the  Lower  Grindelwald  Glacier.  The 
lowering  of  its  surface  has  exposed  two  rock-terraces. 
One  of  these  is  well  glaciated,  showing  roches  mou- 
tonne'es  with  conspicuous  Stoss  and  Lee-Seiten.  Be- 
tween the  mammillated  rocks  stretch  several  shallow 
rock-basins,  some  of  them  being  filled  with  water. 
One  of  these,  according  to  Professor  Penck,  measured 
26  feet  in  breadth,  42  feet  in  length,  and  3^  feet  in 
depth,  and  was  smooth  and  ice-worn  from  end  to  end. 
Both  terraces  are  trenched  by  the  deep  gully  of  the 
Liitschine,  the  upper  portions  of  the  rocky  walls  being 
conspicuously  striated  and  fluted,  while  here  and  there 
they  present  the  shattered  surfaces  which  are  equally 
characteristic  of  glacial  action. 

Professor  Bruckner  has  in  like  manner  described 
the  broken  and  ruptured  rocks  and  smoothed  surfaces 
which  appear  side  by  side  upon  the  bed  of  a  glacier. 


GLACIAL  ACTION  223 

Thus  at  the  Mazellferner  he  saw  resting  upon  the 
jagged  projecting  out-crops  of  certain  rocks  a  block, 
many  cubic  metres  in  size,  enclosed  in  ground-moraine, 
along  with  which  it  had  travelled  over  the  cracked 
and  shattered  rock-ledges.  The  ground-moraine  was 
squeezed  in  between  the  disjointed  masses.  In 
another  place,  where  the  bed-rock  was  well  smoothed 
and  striated,  he  observed  an  irregular  rough  cavity  or 
hollow,  from  which  a  slab  of  rock  had  evidently  been 
extracted.  In  the  recently  deserted  beds  of  the 
Obersalzbachkees  (Hohe  Tauern)  and  the  Hornkees 
(Zillerthal)  he  noticed  that  the  rocks  were  jointed  in 
a  direction  approximately  parallel  to  their  upper  sur- 
face— a  structure  which  has  favoured  their  rupture 
and  displacement.  Here  and  there,  in  the  midst  of  a 
well  smoothed  area,  rough  cavities  indicated  whence 
slabs  had  been  removed  ;  and  now  and  again  the  de- 
tached fragments  themselves  were  detected.  Many 
such  loose  slabs  were  observed  by  the  same  geologist 
on  the  bed  of  the  Stampflkees.  On  one  side  they  ex- 
hibited the  parallel  striation  characteristic  of  rock 
which  has  been  glaciated  in  situ,  while  the  other  sides 
were  rough  and  irregular,  and  showed  no  trace  of 
abrasion.  That  fragments  of  this  character  are  not 
more  frequently  extruded  at  the  lower  end  of  a  glacier 
is  readily  understood  when  we  remember  that  they 
could  not  travel  far  below  ice  without  losing  their 
rough  surfaces,  and  becoming  more  or  less  glaciated 
all  over. 

Professor  Chamberlin  has  recorded  the  occurrence 


224  EARTH  SCULPTURE 

of  similar  phenomena  in  connection  with  some  of 
the  large  tongues  of  ice  which  are  protruded  from  the 
great  "  inland  ice"  of  Greenland.  He  says  :  "  The 
rubbing  of  the  glacier  (Bowdoin  Glacier)  against 
the  shoulders  of  rock  projecting  from  the  side  of  the 
valley  gave  opportunity  for  observing  some  of  the 
special  phenomena  of  such  situations.  At  one  point 
the  process  of  '  plucking '  was  well  indicated  (though 
not  actually  observed)  on  the  lee-slope  of  a  spur  of 
gneissoid  rock.  Blocks  ranging  up  to  three  or  four 
feet  in  width  and  length,  and  one  or  two  feet  in  thick- 
ness had  been  detached  in  considerable  numbers. 
The  process  involved  much  breaking  and  bruising 
with  relatively  little  wear.  Corners  and  angles  were 
broken  off,  and  heavy  bruise  marks  were  observed 
both  on  the  blocks  and  on  the  sides  and  edges  of  the 
cavities  from  which  they  had  been  removed.  At 
some  points  considerable  crushed  rock  was  observed. 
On  the  other  hand,  systematic  grooves  and  strise 
were  not  abundant  nor  pronounced.  The  dynamic 
impression  given  was  that  of  a  forceful  tearing  out  of 
blocks  by  the  action  of  a  relatively  rigid  agency, 
which  did  not  press  the  blocks  hard  upon  the  lee- 
slope  after  their  removal." 

It  is  clear,  then,  that  under  existing  glaciers  and 
ice-fields  rocks  are  sometimes  smoothed  and  polished, 
sometimes  crushed  and  shattered.  The  pressure  of 
the  ice  tends  to  disrupt  rock-masses,  which  yield  or 
resist  according  to  their  character  and  structure,  and 
fragments  detached  must  often  serve  as  wedges  to 


GLACIAL  ACTION  225 

dislocate  and  detach  others.  Nor  can  it  be  doubted 
that  the  rocky  bed  of  a  glacier  is  also  attacked  by 
frost.  The  constant  outflow  of  water  shows  that  in- 
fraglacial  melting  goes  on  all  the  year  round.  The 
temperature  at  the  bottom  of  the  ice  oscillates  about 
the  freezing-point,  and  as  a  glacier  flows  on  its  way 
thawing  and  freezing  must  be  continually  taking  place. 
In  this  way  joints  are  no  doubt  opened,  rock-masses 
loosened,  and  larger  and  smaller  fragments  become 
more  readily  plucked  and  dragged  out  of  place. 

We  cannot,  therefore,  hesitate  to  conclude  that  ice 
in  motion,  whether  in  the  form  of  glaciers  or  of  ice- 
caps, is  a  powerful  agent  of  erosion.  It  not  only 
abrades  and  smooths,  but  breaks  up  and  quarries  the 
rocks  over  which  it  flows,  and  the  ddbris  thus  obtained 
constitutes  the  true  ground-moraine. 

2.  Geological  action  of  prehistoric  glaciers.  Ge- 
ologists rightly  insist  upon  the  potency  of  river-ero- 
sion. The  study  of  modern  denudation  has  quite 
convinced  them  that  valleys  can  be  and  have  been 
excavated  by  running  water.  In  proof  of  this  they 
point  not  only  to  the  present  action  of  rivers— to  the 
rate  of  transport  of  sediment — but  to  the  immense 
accumulations  formed  by  river-action  in  prehistoric 
times.  The  broad  alluvial  plains  of  river-valleys,  the 
great  deltas  which  encroach  upon  the  sea,  the  wide 
stretches  of  flat  lands  occupying  the  sites  of  silted-up 
lakes,  are  all  cited  as  evidence  of  the  potency  of  run- 
ning water  as  a  producer  and  transporter  of  sediment. 
So  in  like  manner  the  glacialist  appeals  to  far-ex- 


226  EARTH  SCULPTURE 

tended  accumulations  of  ground-moraines  as  proof 
of  the  efficiency  of  flowing  ice  as  an  agent  of  erosion 
and  transport. 

The  study  of  modern  glacial  action  is  carried  on 
under  certain  obvious  disadvantages.  The  bed  of  a 
glacier  is  concealed  from  our  view.  Now  and  again 
we  may  get  a  peep  under  the  ice  ;  or,  better  still,  we 
may  have  the  opportunity  of  examining  the  ground 
from  which  a  glacier  has  temporarily  retired.  But 
the  portions  of  a  glacier's  bed  thus  at  times  exposed 
are  not  those  where  erosive  action  is  most  intense. 
A  glacier  thins  away  towards  its  extremity,  and  the 
rate  of  motion  at  the  same  time  diminishes,  so  that 
pressure  and  erosion  must  decrease  with  the  attenua- 
tion of  the  ice.  To  such  an  extent  is  this  the  case, 
that  the  snout  of  a  glacier  deploying  upon  a  rela- 
tively flat  surface  often  rests  upon  its  terminal  mo- 
raines, or  even  overrides  the  fluvio-glacial  gravels 
spread  out  in  front  of  it.  Such  facts  have  led  some 
observers  to  conclude  that  glaciers  do  not  erode  at 
all,  and  did  the  facts  referred  to  stand  alone  there 
would  be  some  justification  for  that  conclusion.  It 
should  be  remembered,  however,  that  were  observers 
of  river-action  to  confine  attention  to  the  broad  plain- 
track — to  the  region  known  as  the  "base-level  of 
erosion" — they  would  no  doubt  readily  come  to  the 
conclusion  that  running  water  transports  and  deposits 
sediment ;  but,  by  following  the  process  of  reasoning 
just  alluded  to,  they  might  also  infer  that  rivers  are 
incapable  of  erosion.  Were  the  beds  of  existing 


GLACIAL  ACTION  227 

glaciers  as  open  to  investigation  as  the  channels  of 
rivers,  we  should  probably  hear  little  about  the  feeble 
erosive  action  of  ice.  But  although  we  cannot  make 
direct  observations  underneath  the  central  and  thicker 
portions  of  a  glacier,  we  can  yet  examine  great  val- 
leys and  broad  lowland  regions  which  have  been 
formerly  subjected  to  intense  glaciation.  And  the 
evidence  of  effective  glacial  erosion  there  displayed 
is  too  clear  to  be  wholly  misunderstood.  Let  us  then 
consider  the  general  results  which  have  been  obtained 
by  the  careful  investigation  of  certain  well  known 
glaciated  regions — the  Alpine  lands  of  Central  Europe. 
At  the  climax  of  the  Glacial  Period  the  snow-line 
in  the  Alps  appears  to  have  been  upon  an  average 
some  4700  feet  lower  than  now.  Viewed  from  the 
north,  the  mountains  must  at  that  time  have  pre- 
sented the  appearance  of  a  great  ice-field,  broken 
here  and  there  by  Nunatakkr — the  protruding  peaks 
of  the  dominant  elevations  of  the  secondary  ranges, 
and  bounded  on  the  south  by  the  snow-clad  ridges  of 
the  Central  Chain.  In  a  word,  so  thick  was  the  ice 
in  the  valleys  that  as  the  glaciers  made  their  way  to 
the  low  grounds  they  frequently  coalesced  or  became 
confluent  across  intervening  mountain-ridges.  Under 
such  conditions  it  is  obvious  that  the  formation  and 
accumulation  of  superficial  moraines  must  have  been 
relatively  limited.  The  area  buried  under  n£v6  and 
ice  was  greatly  in  excess  of  that  which  remained 
uncovered.  If  it  be  true,  therefore,  that  ground- 
moraines  consist  chiefly  of  rock-d<?6rzs  derived  from 


228  EARTH  SCULPTURE 

superficial  sources,  those  of  the  Glacial  Period  should 
be  of  little  importance.  The  very  reverse,  however, 
is  the  case.  The  ground-moraines  assume  an  enorm- 
ous development,  their  dimensions  being  in  direct 
proportion  to  the  size  of  the  ice-flows.  The  larger  the 
body  of  ice,  the  greater  the  mass  of  ground-moraine. 
It  must  be  admitted,  therefore,  that  the  materials 
of  the  old  ground-moraine  cannot  have  been  derived 
from  superficial  sources.  Some  have  suggested,  how- 
ever, that  the  accumulations  in  question  consist  to  a 
large  extent  of  the  products  of  weathering,  of  torren- 
tial and  fluviatile  action,  which  had  gathered  over  the 
mountain-slopes  and  in  the  valleys  before  the  advent 
of  the  Glacial  Period.  There  is  no  reason  to  believe, 
however,  that  rock-rubbish  throughout  the  Alpine 
lands  attained  a  greater  development  at  the  beginning 
of  the  Ice  Age  than  it  does  now.  The  old  snow-fields 
and  glaciers  doubtless  gradually  extended  as  the  tem- 
perature fell.  As  the  depression  of  the  snow-line 
continued,  rock-rubbish  would  accumulate  abundantly, 
just  as  at  present,  in  every  valley  occupied  by  a  gla- 
cier. For  a  long  time,  too,  superficial  moraines  would 
assume  a  relatively  great  importance,  so  that  large 
terminal  moraines  would  mark  every  pause  in  the 
progress  of  the  ice-flows.  But  as  the  glaciers  thick- 
ened in  the  valleys,  and  more  and  more  bare  rock  dis- 
appeared below  the  ice,  the  supply  of  detritus  from 
above  would  become  gradually  limited,  until  in  many 
places,  as  in  the  region  of  the  secondary  ranges,  it 
practically  ceased  altogether.  Were  a  glacial  period 


GLACIAL  ACTION  229 

to  supervene  at  present,- each  individual  glacier  would 
begin  to  advance,  and  as  it  progressed  the  zone  of 
most  active  rock-shattering  by  frost  would  descend 
with  it  to  lower  and  lower  levels.  But  at  each  step 
in  its  advance  the  glacier  would  encounter  no  greater 
accumulations  of  rock-rubbish  than  had  all  along 
gathered  in  its  neighbourhood.  In  short,  as  Dr.  Bohm 
remarks,  weathering  would  proceed  no  more  rapidly 
in  front  of  one  of  the  enormous  glaciers  of  the  Ice 
Age  than  it  does  now  in  the  vicinity  of  existing  gla- 
ciers. "  When  the  Inn  Glacier,"  he  says,  "  had  ad- 
vanced as  far  as  Innsbruck,  it  would  enter  no  zone  of 
more  active  rock-shattering  than  is  met  with  to-day  in 
front  of  the  glaciers  of  the  Oetzthal."  It  is  obvious, 
therefore,  that  if  the  glaciers  of  the  Ice  Age  derived 
their  subglacial  detritus  either  from  above  or  from 
frost-riven  dtbris  and  superficial  deposits  lying  in  their 
path,  their  ground-moraines  could  not  at  any  one 
place  have  attained  a  greater  thickness  than  those  of 
existing  Alpine  glaciers ;  and  yet,  as  is  well  known, 
the  old  ground-moraines  reach  an  astonishing  thick- 
ness, their  bulk  being  in  direct  proportion  to  the  size 
of  the  former  ice-flows. 

One  may  readily  exaggerate  the  importance  of  the 
rock-rubbish  which  is  almost  everywhere  conspicuous 
in  the  Alps.  The  enormous  screes  of  angular  blocks 
and  dtbris  which  shoot  down  from  cliff  and  buttress 
contain  prodigious  quantities  of  materials.  Here,  we 
are  apt  to  think,  is  sufficient  loose  material  where- 
with to  form  ground-moraines  as  thick  and  extensive 


2  30  EARTH  SCULPTURE 

as  those  of  the  Glacial  Period.  But  is  this  actually 
the  case  ?  If  all  the  debris  in  question  could  be  lifted 
and  equally  distributed  over  the  Alpine  lands  it  would 
certainly  not  suffice  to  raise  the  general  surface  of 
those  lands  by  more  than  a  few  feet  or  yards.  The 
old  morainic  accumulations,  on  the  other  hand,  could 
they  be  replaced,  would  add  considerably  to  the  av- 
erage height  of  the  surface.  Professor  Penck  has 
shown,  for  example,  that  the  morainic  accumulations  of 
the  Isar  Glacier  average  a  thickness  of  20  metres,  and 
cover  an  area  of  some  1800  square  kilometres.  They 
have  been  derived  from  an  area  280x3  square  kilometres 
in  extent.  Could  they  be  restored,  therefore,  they 
would  raise  the  general  surface  by  about  13  metres. 
In  other  words,  an  area  of  1081  square  miles  has  been 
lowered  by  some  41  feet.  In  Dr.  Penck's  estimate  only 
the  morainic  matter  has  been  considered,  the  equally 
great  mass  of  fluvio-glacial  gravels  (consisting  almost 
exclusively  of  remodified  infraglacial  detritus)  has 
been  entirely  neglected.  Further,  we  must  remember 
that  during  the  formation  of  the  moraines  and  fluvio- 
glacial  gravel,  enormous  quantities  of  the  fine  flour 
of  rocks — the  result  of  glacial  grinding — must  have 
been  carried  away  in  suspension,  and  deposited  in 
regions  far  beyond  the  glaciated  areas. 

Such  considerations  as  these  show  that  the  old 
morainic  accumulations  cannot  consist  merely  of  the 
superficial  rock-rubbish  which  the  old  glaciers  found 
ready  to  hand,  and  swept  out  as  they  advanced.  All 
such  loose  accumulations,  after  excessive  glacial  con- 


GLACIAL  ACTION  231 

ditions  had  supervened,  must  erelong  have  become 
exhausted,  and  can  form  only  a  small  proportion  of 
the  ancient  ground-moraines.  Whence,  then,  was  the 
great  bulk  of  the  material  derived  ?  Surely  from 
infraglacial  sources,  as  the  direct  result  of  glacial 
erosion.  The  immense  ice-flows  of  the  Glacial  Period 
must  at  an  early  stage  have  completed  the  removal 
of  preglacial  detritus — none  of  that  detritus  can  now 
linger  underneath  any  existing  glacier,  either  in  the 
Alps  or  in  Norway.  Yet,  as  we  have  seen,  ground- 
moraines  are  forming  at  present  in  both  regions.  In 
the  Alps,  according  to  Professor  Heim  and  others,  the 
ground-moraines  are  fed  from  the  surface,  but  this 
can  be  true  to  only  a  very  limited  extent.  The  pla- 
teau ice-sheets  of  Norway  carry  no  superficial  detritus, 
and  their  ground-moraines  are,  therefore,  supposed 
by  some  to  represent  the  rock-rubbish  which  gathered 
over  the  Scandinavian  heights  in  preglacial  times ! 
A  vast  ice-sheet,  as  we  know,  overflowed  those  re- 
gions during  the  Glacial  Period,  and  buried  the  low 
grounds  to  great  depths  under  the  detritus  which 
it  carried  outwards  from  the  mountains,  and  yet  we 
are  to  believe  that  much  loose  rock-rubbish  of  pre- 
glacial age  still  remains  to  be  removed  from  the  con- 
tinuously ice-covered  plateaux  of  Norway  !  Must  we 
likewise  believe  that  the  "inland  ice"  of  Greenland, 
which  has  probably  persisted  since  Pliocene  times, 
has  not  yet  succeeded  in  removing  the  products  of  sub- 
aerial  weathering,  which  came  into  existence  before 
glacial  conditions  had  supervened  in  Arctic  regions? 


CHAPTER  XI 

LAND-FORMS  MODIFIED  BY  GLACIAL  ACTION 
(Continued) 

FORMER  GLACIAL  CONDITIONS  OF  NORTHERN  EUROPE — EXTENT 
OF  THE  OLD  INLAND  ICE GENERAL  CHARACTER  OF  BOULDER- 
CLAY CENTRAL  REGION  OF  GLACIAL  EROSION  AND  PERIPH- 
ERAL AREA  OF  GLACIAL  ACCUMULATION — FLUVIO-GLACIAL 
DEPOSITS — LOESS,  ORIGIN  OF  ITS  MATERIALS — GLACIATION  OF 
NORTH  AMERICA — MODIFICATIONS  OF  SURFACE  PRODUCED  BY 
GLACIAL  ACTION. 

IF  a  study  of  the  glacial  and  fluvioglacial  deposits 
of  the  Alpine  lands  leaves  us  in  no  doubt  as 
to  the  efficiency  of  glacial  erosion,  an  investigation 
of  the  similar  accumulations  of  Northern  Europe 
and  North  America  is  even  more  convincing.  The 
boulder-clays  of  those  wide  regions  are  true  ground- 
moraines,  recalling  in  every  particular  the  ground- 
moraines  of  the  Alpine  lands.  At  the  climax  of  the 
Glacial  Period  a  great  ice-sheet  covered  all  Northern 
and  North-western  Europe,  extending  east  from  the 
British  area  to  the  Timan  mountains,  and  south  to 
the  German  ranges.  The  ice-sheet  thus  occupied  an 
area  of  2,500,000  square  miles  or  thereabout  in  extent. 
Above  the  surface  of  this  inland  ice  peered  some  of 

232 


GLACIAL  ACTION  233 

the  loftier  mountain-tops  of  Scandinavia,  and  a  few 
Nunatakkr  in  the  British  Islands.  In  the  low  grounds 
of  Scotland  the  sheet  could  hardly  have  averaged  less 
than  2500  to  3000  feet  in  thickness.  In  some  of  the 
Norwegian  fiords  it  exceeded  5500  feet.  Taking  the 
elevation  of  the  ice-shed  in  Scandinavia  as  7000  feet, 
and  the  height  reached  by  the  ice-front  upon  the 
northern  slopes  of  the  mountains  of  Germany  as  1350 
feet,  we  get  a  thickness  for  the  ice-sheet  in  South 
Sweden  of  2900  feet,  of  2500  feet  in  Denmark,  and 
of  1300  feet  or  thereabout  in  the  neighbourhood  of 
Berlin. 

It  is  obviously  impossible  that  the  ground-moraines 
of  an  ice-sheet  of  such  dimensions  could  have  been 
derived  or  even  supplemented  to  any  extent  from 
superficial  sources.  The  boulder-clays  are  the  direct 
products  of  glacial  erosion.  They  consist  essentially 
of  unweathered  material.  Boulders,  smaller  stones, 
grit,  sand,  and  the  finer-grained  rock-meal  or  flour  are 
all  alike  fresh  ;  they  have  not  been  altered  chemically 
as  they  would  have  been  had  they  come  from  super- 
ficial sources.  They  could  not  have  been  derived 
from  above,  and  they  cannot  represent  the  weathered 
fock-afebrts  of  preglacial  times. 

The  external  configuration  assumed  by  boulder- 
clay  seems  likewise  to  point  to  the  infraglacial  origin 
of  the  deposit.  In  relatively  narrow  mountain-valleys 
it  forms  broad  terraces  or  platforms — now  trenched 
and  furrowed  by  streams  and  rivers.  In  broad  low- 
land tracts,  as  in  Tweeddale  and  Nithsdale,  it  is  ar- 


234  EARTH  SCULPTURE 

ranged  in  parallel  banks,  mounds,  and  ridges,  the 
longer  axes  of  which  coincide  with  the  trend  of  glaci- 
ation.  Over  wide  plains,  on  the  other  hand,  it  rises 
and  falls  in  long,  gentle  swellings.  This  varying  con- 
figuration is  undoubtedly  original — it  is  not  the  result 
of  subsequent  subaerial  erosion.  In  mountain-valleys 
the  ice-flow,  subject  to  no  deflection,  must  have  pro- 
ceeded continuously  in  one  direction,  and  its  ground- 
moraine,  we  may  suppose,  would  thus  tend  to  accrete 
more  or  less  regularly.  In  the  broader  lowland  tracts, 
however,  as  in  the  lower  reaches  of  Nithsdale,  Teviot- 
dale,  and  Tweeddale,  the  same  uniformity  of  condi- 
tions did  not  exist.  Each  of  these  broad  depressions 
was  occupied  by  mers  de  glace,  formed  by  the  conflu- 
ence of  ice-flows  streaming  out  from  various  ice-sheds. 
Under  such  conditions  the  movement  of  the  united 
currents  could  not  be  so  equable,  and  in  consequence 
of  variations  in  the  pressure  of  the  ice,  and  in  the 
lines  of  most  rapid  motion,  the  ground-moraine  would 
tend  to  heap  up  in  banks  or  ridges,  the  longer  axes 
of  which  would,  necessarily  coincide  with  the  direc- 
tion of  ice-flow.1 

1  The  "drumlins"  and  "drums  "of  Ireland  and  Scotland  appear  to  be  repre- 
sented in  Sweden  by  certain  banks  of  boulder-clay,  which  are  described  by  De 
Geer  as  a  novel  kind  of  radical  moraines.  He  recognises  their  strong  resem- 
blance to  the  drumlins  of  New  England  (Geol.  Foren.  Fork.,  1895,  p.  212). 
Drumlins  occur  in  the  Island  of  Rtigen,  but  they  would  seem  to  be  rare  in  North 
Germany.  Recently  Dr.  K.  Keilhack  has  observed  them  in  Neumark  (Jahrb., 
d.  konigl.  preuss.  geol.  Landesanstalt  fur  1893,  1895,  p.  190).  They  have 
been  recognised  also  in  the  low  grounds  of  Switzerland  by  Dr.  Fruh  {Jahres- 
bericht  d.  St.  Gallischen  Naturwis sense h.  Ges.,  1894-95).  It  is  probable,  how- 
ever, that  the  lenticular  mounds  and  banks  of  till  known  under  the  name  of 
drumlins  have  not  all  been  formed  in  the  same  way.  Thus  the  short  lenticular 


GLACIAL  ACTION  235 

Once  more,  over  the  peripheral  areas  of  the  inland 
ice,  as  in  the  great  plains  of  Germany,  the  influence 
exerted  by  the  confluence  of  ice-flows  just  referred  to 
would  no  longer  be  felt,  at  least  to  the  same  extent. 
When  the  ice  had  fairly  escaped  from  uplands  and 
hilly  ground  all  minor  movements  would  merge  in 
one  continuous  broad  outflow,  the  ground-moraine, 
as  a  result,  being  spread  out  more  or  less  uniformly. 

Looked  at  broadly,  Northern  Europe  displays  a 
central  region  of  glacial  erosion  and  a  peripheral  area 
of  glacial  accumulation.  In  the  former,  as  in  the 
Scandinavian  peninsula,  Finland,  and  the  more  ele- 
vated portions  of  the  British  Islands,  bare  rock  is 
conspicuous  over  wide  districts,  while  glacial  accumu- 
lations, confined  for  the  most  part  to  hollows  and 
depressions,  attain  as  a  rule  no  great  thickness.  Out- 
side of  such  areas  of  special  erosion,  on  the  other 
hand,  as  in  the  low  grounds  of  England  and  the  plains 
of  Northern  Europe,  naked  rock  appears  only  at 
intervals,  while  morainic  materials  and  fluvio-glacial 
deposits  reach  their  greatest  development. 

Under  the  ice-sheet  rock-grinding  and  rock-shatter- 
ing were  carried  on  side  by  side.  No  doubt  the 
boulder-clays  frequently  rest  upon  a  smoothed  and 

drumlins  of  South  Galloway  appear  to  owe  their  origin  to  glacial  erosion. 
They  are  the  relics  of  the  sheet  of  boulder-clay  which  accumulated  under  the 
last  general  mer  de  glace  that  overwhelmed  Scotland.  At  a  later  stage  the 
Southern  Uplands  supported  local  ice-sheets  and  large  glaciers  which,  flowing 
out  upon  the  adjacent  low  grounds,  ploughed  into  and  greatly  denuded  the  old 
boulder-clay.  The  drumlins  of  this  region  are,  in  short,  simply  roches 
moutonn&s,  composed  sometimes  entirely  of  boulder-clay,  at  other  times  partly 
of  boulder-clay  and  partly  of  solid  rock. 


236  EAR  TH  SCULP  TURE 

striated  surface,  but  just  as  frequently  the  ground- 
rock  is  shattered,  crushed,  and  jumbled,  and  the 
cttbris  mixed  up  with  the  overlying  till.  Such  phe- 
nomena are  not  confined  to  any  particular  area. 
Examples  of  finely  smoothed  and  of  jumbled  rock- 
surfaces  may  often  be  seen  in  one  and  the  same 
quarry  or  other  opening.  The  latter,  however,  are 
best  developed  in  places  where  the  ground-rock 
tended  to  yield  most  readily  to  the  pressure  of  ice. 
Massive  crystalline  rocks  are  perhaps  oftener  smoothed 
than  shattered  below  till  ;  but  again  and  again  their 
jointed  structure  has  led  to  their  ready  disruption, 
boulder-clay  has  been  squeezed  into  their  crevices, 
and  numerous  blocks,  some  of  large  size,  have  been 
torn  out  and  enclosed  in  the  till.  The  result  of  this 
infraglacial  disruption,  however,  is  better  seen  in  the 
case  of  bedded  rocks,  especially  when  the  dip  of  the 
strata  has  happened  to  coincide  with  the  direction  of 
ice-flow.  In  such  cases  the  boulder-clay  has  often 
been  forced  in  between  the  bedding-planes,  and  broad 
ledges  and  reefs  of  rock  have  been  wedged  up  and 
forced  out  of  place.  Not  only  so,  but  in  the  case  of 
chalk  and  certain  Tertiary  formations,  the  pressure 
of  the  ice-sheet  has  not  infrequently  squeezed  the 
rocks  into  folds  and  flexures  of  such  a  character  that 
the  disturbance  and  contortion  have  sometimes  been 
attributed  to  subterranean  action.  Superficial  curv- 
ing, flexing,  and  displacement  of  the  kind  referred  to 
are  met  with  both  in  high  and  low-lying  regions  ;  but 
as  the  more  yielding  strata  are  best  developed  within 


GLACIAL  ACTION  237 

the  latter,  it  is  there  that  we  meet  with  the  most 
striking  evidence  of  infraglacial  disruption  and  quar- 
rying. 

From  the  various  facts  above  referred  to  we  are 
justified  in  concluding  that  glacier-ice  is  a  most  effect- 
ive agent  of  erosion.  It  not  only  abrades,  rubs, 
smooths,  and  polishes,  but  crushes,  folds,  disrupts, 
and  displaces  rock-masses,  the  amount  of  disturbance 
being  in  proportion  to  the  resisting  power  of  the 
rocks  and  the  pressure  exerted  by  the  ice.  Other 
things  being  equal,  more  crushing  and  displacement 
will  be  effected  under  a  massive  ice-sheet  than  under 
a  small  valley-glacier.  It  is  obvious,  therefore,  that 
during  the  prolonged  existence  of  an  ice-sheet,  trans- 
port and  accumulation  must  result  in  very  consider- 
able modifications  of  the  surface.  The  central  area 
of  dispersion  becomes  gradually  lowered  by  the  ab- 
straction of  rock-afe&ris  which  is^  carried  forward  and 
accumulated  over  the  peripheral  area  occupied  by  the 
mer  de  glace.  Hence  it  is  that  in  the  former  region 
ground-moraines  are  seldom  very  thick,  and  usually 
consist  of  local  materials.  As  they  are  followed  out- 
wards, however,  they  gradually  attain  a  greater  depth, 
and  are  more  widely  spread,  the  local  materials 
becoming  more  and  more  mixed  with  far-travelled 
detritus,  until  eventually  the  latter  begins  to  predom- 
inate. The  depth  attained  by  the  ground-moraines 
in  the  plains  of  Europe  is  often  great,  individual 
sheets  of  boulder-clay  often  exceeding  one  hundred 
feet  in  thickness. 


238  EARTH  SCULPTURE 

Such  boulder-clays,  however,  are  not  the  only  evid- 
ence of  glacial  erosion.  With  them  are  frequently 
associated  beds  of  gravel  and  sand  and  laminated  clay, 
consisting  exclusively  of  erratic  materials.  These 
are  admittedly  the  products  of  infraglacial  water- 
action  ;  the  materials  have  been  derived  principally, 
if  not  exclusively,  from  the  washing  and  sifting  of 
infra-  and  intra-glacial  detritus.  Extensive  beds  of 
such  aqueous  accumulations  underlie  the  ground- 
moraines  in  some  places,  and  in  other  places  separate 
one  mass  of  ground-moraine  from  another.  Great 
mounds,  banks,  and  sheets  of  the  same  character, 
which  obviously  are  similar  in  origin  to  the  fluvio- 
glacial  detritus  of  the  Alpine  Vorlander,  fringe  the 
margins  of  the  ground-moraines,  and  sweep  over 
wide  areas  in  North  Germany  and  Russia.  All  these, 
therefore,  must  be  taken  account  of  if  we  would  form 
an  adequate  conception  of  the  amount  of  erosion 
effected  by  the  mers  de glace  of  the  Ice  Age. 

The  diluvial  deposits  of  North  Germany  necessarily 
vary  in  thickness.  Sometimes  they  are  only  a  few 
feet,  at  other  times  they  exceed  200  yards.  Dr. 
Wahnschaffe  has  collected  the  results  of  numerous 
borings  made  in  those  regions,  from  which  we  learn 
that  in  East  Prussia  they  range  in  thickness  from  20 
feet  up  to  490  feet,  in  West  Prussia  from  20  feet  to 
360  feet,  in  Posen  from  35  feet  to  240  feet,  in  Bran- 
denburg from  30  feet  to  670  feet,  in  Mecklenburg 
from  6  feet  to  430  feet ;  in  the  province  of  Saxony  a 
depth  of  400  feet  has  been  noted.  Mr.  Amund  Hel- 


GLACIAL  ACTION  239 

land,  after  conferring  with  geologists  to  whom  the 
diluvial  accumulations  of  the  great  plains  are  familiar, 
comes  to  the  conclusion  that  the  deposits  probably 
attain  an  average  thickness  of  150  feet.  The  ma- 
terials being  partly  of  local  and  partly  of  foreign  ori- 
gin, he  deducts  the  former  (estimated  at  50  feet), 
and  thus  obtains  a  thickness  of  100  feet  for  the 
detritus  derived  from  Sweden  and  Finland,  and  spread 
over  the  low  grounds  of  North  Germany,  etc.  Ac- 
cording to  this  geologist,  the  glaciated  areas  of 
Sweden  and  Finland,  which  supplied  the  detritus,  are 
some  800,000  square  kilometres  in  extent  (497,120 
square  miles),  while  the  area  in  Russia  and  North 
Germany  over  which  Swedish  and  Finnish  erratic 
materials  are  spread  is  estimated  at  2,040,000  square 
kilometres  (1,267,656  square  miles).  Were  those 
materials  therefore  transferred  to  the  lands  from  which 
they  have  been  derived,  they  would  raise  the  general 
surface  by  255  feet.  This  estimate,  it  need  hardly  be 
said,  is  a  mere  rough  approximation,  and  is  probably  ex- 
cessive. But  even  if  it  be  supposed  that  Helland  has 
exaggerated  both  the  amount  of  foreign  erratic  ma- 
terials and  the  extent  of  the  area  over  which  it  is  dis- 
tributed, we  shall  still  be  compelled  to  admit  that  the 
surface  of  Scandinavia  must  have  been  greatly  modi- 
fied by  glacial  erosion.  If  we  deduct  two-thirds  from 
Helland's  result  we  have  still  left  sufficient  material 
to  raise  the  general  surface  of  Finland  and  Sweden 
by  85  feet. 

In  the  following  chapter  reference  is  made  to  the 


240  EARTH  SCULPTURE 

loss  as  being  primarily  a  flood-loam  of  glacial  times. 
Much  of  that  occurring  in  the  river-valleys  of  Central 
Europe  has,  no  doubt,  been  derived  from  the  Alpine 
lands  ;  but  the  vast  accumulations  of  loss  in  Southern 
and  South-eastern  Russia  doubtless  owe  their  origin 
chiefly  to  the  flood-waters  escaping  from  the  margins 
of  the  old  "  inland  ice."  All  these  deposits,  as  we  shall 
see,  have  been  more  or  less  rearranged  and  modified 
by  subaerial  action,  but  the  materials  themselves 
would  seem  to  have  resulted,  in  largest  measure  at 
least,  from  the  washing  and  weathering  of  glacial  ac- 
cumulations. In  short,  they  are  additional  evidence 
of  the  effective  erosive  action  of  flowing  ice. 

The  researches  of  geologists  in  North  America  are 
on  all  fours  with  those  carried  on  in  Europe.  They 
tell  precisely  the  same  tale.  The  American  boulder- 
clays,  fluvio-glacial  gravels,  and  loss  present  us  with 
similar  phenomena.  As  in  Europe  so  in  North 
America,  broken  and  ruptured  rocks  are  of  common 
occurrence  under  the  overlying  ground-moraines. 
The  ice-sheet,  as  Dana  remarks,  "  carried  debris  for 
the  most  part,  not  from  the  slopes  and  summits  of 
emerged  ridges,  but  from  those  underneath  it.  ... 
It  obtained  its  load  by  abrading,  ploughing,  crushing, 
and  tearing  from  those  underlying  slopes  and  sum- 
mits. .  .  .  The  ice-mass  was  a  coarse  tool  ;  but 
through  the  facility  with  which  it  broke  and  adapted 
itself  to  uneven  surfaces,  it  was  well  fitted  for  all 
kinds  of  shoving,  tearing,  and  abrading  work.  More- 
over it  was  a  tool  urged  on  by  enormous  pressure. 


GLACIAL  ACTION  241 

A  thickness  of  looofeet  corresponds  to  at  least  50,000 
pounds  to  the  square  foot.  The  ice  that  was  forced 
into  the  openings  and  crevices  in  the  rocks  had 
thereby  enormous  power  in  breaking  down  ledges, 
prizing  off  boulders,  and  in  abrading  and  corroding." 

3.  Modifications  of  the  surface  produced  by  glacial 
action.  Having  now  learned  that  glacier-ice  is  a  most 
effective  eroding  agent,  we  have  next  to  consider  the 
modifications  of  the  land-surface  brought  about  by 
glacial  action.  Looked  at  broadly,  as  we  have  seen, 
each  glaciated  region  shows  a  central  area  of  erosion 
and  a  peripheral  area  of  accumulation.  Not  that 
erosion  and  accumulation  are  confined  in  this  way 
each  to  a  separate  tract,  but  simply  that  in  the  central 
area  erosion  is  in  excess  of  accumulation,  while  in  the 
surrounding  region  the  reverse  is  the  case.  It  will 
conduce  to  clearness,  therefore,  if  we  consider  first  the 
characteristic  features  which  are  the  direct  result  of 
glacial  erosion.  Thereafter  we  shall  glance  at  the 
aspect  presented  by  a  land  more  or  less  covered  with 
glacial  and  fluvio-glacial  detritus. 

Unquestionably  the  most  notable  features  of  a 
well  glaciated  country  is  its  rounded  and  flowing 
configuration,  a  configuration  which  is  always  most 
striking  when  viewed  in  the  direction  of  glaciation. 
Tors,  peaks,  buttresses,  and  ridges  have  been  smoothed 
down,  escarpments  bevelled  off,  and  asperities  in 
general  softened.  This  is  the  direct  result  of  glacial 
abrasion,  but  accumulation  also  has  helped  in  the 
production  of  a  flowing  contour,  for  many  of  the 


242  EARTH  SCULPTURE 

dimples  and  smooth  depressions  upon  hill-tops  and 
hill-slopes  are  more  or  less  due  to  glacial  deposition. 
While  projecting  rock-masses  have  been  abraded  and 
removed,  irregular  hollows,  gullies,  ravines,  and  other 
rough  depressions  have  often  been  partially  or  com- 
pletely obliterated  by  the  deposition  in  them  of 
morainic  materials,  abrasion  and  accumulation  to- 
gether having  thus  resulted  in  the  production  of  a 
more  or  less  undulating  surface.  In  the  phenomena 
of  "  crag  and  tail "  we  see  another  effect  of  the  same 
twofold  action.  Isolated  stacks  and  bastions  of  rock, 
which  faced  the  direction  of  ice-flow,  have  been 
rounded  and  bevelled-off,  and  frequently  a  hollow 
dug  out  in  front,  while  morainic  dtbris  has  been 
heaped  up  behind  to  form  the  so-called  u  tail "  of  the 
hill.  There  are  endless  modifications  of  this  structure. 
Thus  in  many  hilly  tracts  which  have  been  completely 
overwhelmed  by  an  ice-flow  we  may  often  trace  series 
of  parallel  ridges  and  intervening  hollows  of  various 
width,  height,  and  depth,  which  obviously  extend  in 
the  direction  of  former  glaciation.  These  are  the 
result  partly  of  erosion  and  partly  of  accumulation. 
The  hollows  show  where  the  rock  has  most  readily 
yielded  to  glacial  erosion,  while  the  ridges  consist  of 
irregular-shaped  masses  and  ledges  of  more  durable 
rocks,  and  of  morainic  material  which  has  gathered 
in  their  rear.  Into  these  and  other  details  of  glacial 
action,  however,  it  is  not  necessary  to  go.  For  our 
purpose  it  is  enough  to  recognise  the  general  fact 
that  glaciation  tends  to  obscure  and  obliterate  the 


GLACIAL  ACTION  243 

features  which  result  from  the  action  of  the  ordinary 
agents  of  erosion  and  denudation.  Hence  all  well- 
glaciated  areas  show  a  somewhat  monotonous  outline 
—round-backed  rocks,  smoothed  and  undulating  hill- 
slopes  and  hill-tops, — in  a  word,  undulating  contours 
are  everywhere  conspicuous. 

The  effect  produced  by  glacial  action  is  perhaps 
most  strikingly  displayed  in  regions  the  more  elevated 
portions  of  which  have  risen  above  the  surface  of  the 
ice,  and  so  escaped  abrasion.  In  the  great  valleys  of 
the  Alps,  for  example,  how  strongly  contrasted  are 
the  glaciated  and  non-glaciated  areas  !  In  the  Upper 
Engadine  the  valley  slopes  up  to  a  height  of  2000 
feet  or  thereabout  are  conspicuously  abraded,  while 
above  that  level  all  is  harsh  and  rugged.  It  is  the 
same  in  our  own  islands,  as,  for  example,  in  the  Outer 
Hebrides,  where  the  whole  area  is  smoothed  and 
rounded  up  to  a  height  of  1500  or  1600  feet,  above 
which  level  the  rocks  present  quite  a  different  aspect. 

But  glacier-ice  does  not  only  abrade  and  bevel-off 
prominent  rock-ledges,  peaks,  tors,  bastions,  and  but- 
tresses, it  also  excavates  hollows,  which  may  vary  in 
extent  from  a  few  feet  or  yards  in  depth  and  width  to 
great  depressions  measuring  many  fathoms  deep  and 
not  a  few  miles  in  extent.  Here,  however,  we  come 
upon  the  vexed  question  of  the  origin  of  rock-basins, 
the  consideration  of  which  may  be  conveniently  de- 
ferred for  the  present. 

The  transfer  of  detritus  from  the  area  of  dominant 
glacial  erosion,  and  its  distribution  over  the  peripheral 


244  EARTH  SCULPTURE 

area  of  dominant  accumulation,  has  very  considerably 
modified  the  aspect  of  the  land.  Could  we  remove 
all  glacial  deposits  from  our  own  broad  lowland  val- 
leys, it  is  certain  that  the  sea  would  in  many  places 
penetrate  far  inland.  On  the  continent  the  Baltic 
would  overflow  wide  tracts  in  the  plains  of  Northern 
Germany,  for  the  bottom  of  the  deposits  of  that  re- 
gion descends  frequently  below  the  level  of  the  sea. 
And  similar  changes  would  be  brought  about  were  the 
glacial  accumulations  of  North  America  to  disappear 
- — the  sea  would  encroach  upon  the  land.  Very  con- 
siderable modifications  were  likewise  effected  in  the 
drainage-systems  of  extensive  regions.  In  Europe 
and  North  America  alike,  the  irregular  deposition  and 
distribution  of  glacial  and  fluvio-glacial  accumulations 
have  often  led  to  remarkable  changes  in  the  directions 
followed  by  the  streams  and  rivers,  which  reappeared 
as  the  great  mers  de glace  melted  away.  Throughout 
the  peripheral  areas  of  dominant  deposition  preglacial 
courses  and  channels  were  largely  filled  up  with  de- 
tritus, and  not  infrequently  had  become  in  this  way 
obliterated,  so  that  the  streams  and  rivers  of  post- 
glacial times  were  often  deflected  and  compelled  to 
erode  new  channels. 

It  is  not  with  such  changes,  however,  that  we  are 
at  present  concerned,  but  rather  with  the  various 
forms  assumed  by  glacial  accumulations.  Ground- 
moraines,  as  we  have  already  seen,  present  certain 
typical  configurations.  And  the  same  is  true  of  lat- 
eral and  terminal  moraines,  and  of  fluvio-glacial  de- 


GLACIAL  ACTION  245 

posits.  In  areas  of  dominant  glacial  accumulation, 
as  in  Schleswig-Holstein  and  North  Germany,  the 
ground-moraines  often  occupy  the  surface  over  exten- 
sive regions,  and  form  wide  plains  with  a  softly  undu- 
lating surface.  The  ground  rises  and  falls  gently  in 
long,  broad  swellings  and  depressions,  which  do  not 
seem  to  follow  any  particular  direction.  In  other  re- 
gions, as  in  the  Lothians  and  elsewhere  in  our  own 
lowlands,  the  undulations  of  the  boulder-clay  not  in- 
frequently show  a  rudely  parallel  arrangement.  Ever 
and  anon,  however,  all  traces  of  definite  orientation 
disappear,  and  the  ground  then  simply  rises  and  falls 
irregularly  as  in  the  plains  of  North  Germany.  But 
in  some  of  the  broader  dales  of  Scotland  the  config- 
uration of  the  boulder-clay  becomes  strongly  defined, 
the  accumulation  being  arranged  in  a  well  marked 
series  of  long  parallel  banks  known  as  "  drums "  or 
"  sowbacks."  Elsewhere,  again,  as  in  Galloway  and 
in  many  parts  of  Ireland,  the  ground-moraines  often 
assume  the  form  of  short  or  more  or  less  abrupt  len- 
ticular hills,  or  "  drumlifls,"  as  they  are  termed. 

Another  set  of  characteristic  glacial  land-forms  are 
the  eskers,  or  osar.  These  are  somewhat  abrupt 
banks  and  ridges  of  gravel  and  sand,  which  are  be- 
lieved to  have  been  formed  in  tunnels  underneath  the 
great  mers  de  glace.  They  are  well  seen  in  certain 
tracts  of  our  own  islands,  but  reach  their  greatest  de- 
velopment in  Sweden,  where  they  traverse  the  land  as 
great  embankments,  rising  to  a  height  of  50  or  100 
feet  above  the  general  level,  and  following  a  sinuous 


246  EARTH  SCULPTURE 

or  river-like  course  for  distances   of  sometimes   150 
miles  or  more. 

Other  hillocks  and  hills  of  glacial  origin  are  lateral 
and  terminal  moraines.  The  former  are  practically 
confined  to  mountain-valleys,  while  the  latter  are  met 
with,  not  only  in  mountain-valleys,  but  in  lowlands 
often  far  removed  from  any  elevated  region.  In 
mountain-valleys  such  moraines  consist  chiefly  of  an- 
gular rock-debris,  but  in  low  grounds  opposite  the 
mouths  of  mountain-valleys  they  are  usually  com- 
posed more  largely  of  ground-moraine,  together  with 
gravel  and  sand  and  a  certain  admixture  of  angular 
debris  and  blocks,  sometimes  the  one  and  sometimes 
the  other  kind  of  material  predominating.  In  Eu- 
rope, the  most  remarkable  terminal  moraines  are 
those  which  denote  the  limits  reached  by  the  glaciers 
and  ice-sheets  of  the  Glacial  Period.  They  are  strongly 
developed  in  the  Vor lander  of  the  Alps,  in  Southern 
Scandinavia,  Schleswig-Holstein,  North  Germany, 
and  Finland  ;  and  on  a  smaller  scale  they  abound  in 
our  own  islands.  Looked  at  broadly,  such  moraines 
occur  as  more  or  less  abrupt  mounds  and  crescent- 
like  or  undulating  ridges.  Opposite  the  mouths  of 
important  mountain-valleys  they  are  often  disposed 
in  concentric  series,  one  or  more  dominant  lines  of 
banks  and  ridges  with  many  subordinate  hummocks, 
heaps,  and  irregular  low  mounds  lying  behind  and 
between  them.  Not  infrequently  they  present  a  most 
tumultuous  appearance — cones,  mounds,  banks,  and 
ridges  confusedly  heaped  together,  and  thus  enclosing 


GLACIAL  ACTION  247 

multitudinous  hollows  and  depressions  of  all  shapes 
and  sizes,  many  of  which  contain  lakes  or  pools,  while 
others  are  occupied  by  bogs  or  simply  clothed  with 
grass  and  herbage.  The  hillocks  and  ridges  vary 
much  in  height  and  size,  among  the  most  conspicuous 
being  those  of  Piedmont  and  Lombardy,  where  they 
occasionally  attain  the  exceptional  elevation  of  more 
than  a  thousand  feet  above  the  adjacent  low  grounds. 
More  usually  in  the  Alpine  Vorldnder  they  do  not  ex- 
ceed two  or  three  hundred  feet.  The  terminal  mor- 
aines of  the  great  Baltic  Glacier  in  Finland,  North 
Germany,  Denmark,  and  Southern  Sweden  present 
much  the  same  appearance  as  those  of  the  Alpine 
Vorldnder.  The  most  conspicuous  are  those  which 
mark  the  extreme  limits  reached  by  that  great  ice- 
stream.  These  rise  more  or  less  abruptly  above  the 
level  of  the  broad  plains  of  gravel,  sand,  and  boulder- 
clay  which  sweep  outwards  from  their  base  into  the 
low  ground  of  North  Germany  and  Poland.  The  land 
lying  between  those  external  ridges  and  the  shores  of 
the  Baltic  forms  a  typical  paysage  moramique — wide 
plains  traversed  now  and  again  by  winding  irregular 
ridges  of  gravel  and  sand,  and  more  or  less  abundantly 
sprinkled  with  mounds  and  banks  of  similar  materials. 
Here  and  there  these  hillocks  crowd  more  closely  to- 
gether, giving  rise  to  a  tumultuously  undulating  sur- 
face ;  while  in  other  places  they  are  drawn  out  in 
curving  lines  and  belts,  or  bands.  Throughout  the 
whole  area  shallow  lakes  and  lakelets,  bogs,  and  mo- 
rasses are  abundantly  developed.  The  surface  of  the 


248  EARTH  SCULPTURE 

flat  lands  lying  within  this  great  morainic  tract  is 
usually  formed  superficially  of  fluvio-glacial  deposits, 
and  the  same  is  the  case  generally  with  the  low  grounds 
immediately  outside  of  the  pay  sage  morainique. 

To  sum  up  the  general  results  of  glacial  action,  we 
may  say  that  this  action  is  entirely  mechanical.  Un- 
der the  influence  of  ordinary  weathering  each  par- 
ticular kind  of  rock  tends  to  assume  a  more  or  less 
characteristic  outline.  With  glaciation,  however,  this 
is  not  the  case.  All  rocks  subjected  to  glacial  action 
become  abraded  after  one  and  the  same  fashion. 
The  tendency  of  that  action  is  to  reduce  asperities,  to 
smooth  and  flatten  the  surface.  But  glacial  action  has 
usually  been  arrested  long  before  its  work  has  been 
completed.  It  is  only  here  and  there  that  projecting 
rocks  have  been  ground  away  and  reduced  to  a  plain 
surface.  In  most  cases  they  are  simply  rounded  off, 
and  so  rocky  hill-slopes  tend  to  assume  mammiform 
outlines.  Some  rocks  are,  of  course,  more  readily  re- 
duced than  others  ;  but  whether  the  rocks  be  hard  or 
soft,  they  all  acquire  the  same  undulating  configura- 
tion. In  regions  of  dominant  glacial  erosion  the 
rounded  and  undulating  surface  is  often  in  part  due 
to  glacial  accumulation,  the  abrupt  depressions  of  the 
ground  being  not  infrequently  filled  up  and  replaced 
by  smoothly  outlined  hollows. 

Where  the  region  of  glacial  erosion  merges  into 
that  of  glacial  deposition,  it  is  often  hard  to  say 
whether  morainic  matter  or  solid  rock  enters  more 
largely  into  the  formation  of  the  banks  and  hillocks 


GLACIAL  ACTION 


249 


that  extend  outwards  from  the  base  of  the  mountain- 
Eventually,  however,  we  pass  into  the  region 


area. 


of  dominant  accumulation — the  region  of  ground- 
moraines  and  eskers,  of  terminal  moraines,  lakes,  and 
fluvio-glacial  plains. 


CHAPTER  XII 

LAND-FORMS  MODIFIED  BY  ^OLIAN  ACTION 

INSOLATION  AND  DEFLATION  IN  THE  SAHARA — FORMS  ASSUMED 
BY  GRANITOID  ROCKS  AND  HORIZONTAL  AND  INCLINED 
STRATA REDUCTION  OF  LAND-SURFACE  TO  A  PLAIN FORM- 
ATION OF  BASINS DUNES  OF  THE  DESERT SAND-HILLS  OF 

OTHER    REGIONS — TRANSPORT     AND  ACCUMULATION    OF    DUST 

LOESS,    A    DUST     DEPOSIT LAKES     AND     MARSHES     OF     THE 

STEPPES. 

AT  the  outset  of  our  inquiry  into  the  origin  of  sur- 
face features,  we  briefly  considered  the  general 
nature  of  the  work  done  by  the  principal  epigene 
agents.  We  saw  that  these  agents  are  often  so 
closely  associated  in  their  operations  that  their  in- 
dividual share  in  the  final  result  can  hardly  be  de- 
termined. In  our  country,  for  example,  erosion  is 
effected  by  the  combined  action  of  the  atmosphere, 
of  frost,  and  of  rain  and  running  water.  There  are 
many  regions,  however,  in  which  one  or  other  of 
these  agents  is  by  far  the  more  conspicuous  worker. 
Thus,  at  lofty  elevations  in  temperate  regions,  and 
throughout  the  higher  latitudes,  the  most  potent 
causes  of  rock-disintegration  and  removal  are  frost, 
snow,  and  ice.  In  warm-temperate,  subtropical,  .and 

250 


AEOLIAN  ACTION  251 

tropical  lands,  on  the  other  hand,  it  is  usually  the 
chemical  and  mechanical  action  of  the  rain  and  run- 
ning water  which  impresses  the  observer,  while  in 
rainless  and  desiccated  regions  insolation  and  defla- 
tion play  the  most  important  role.  It  is  in  the  latter, 
therefore,  that  the  erosive  action  of  wind  is  best 
displayed.  Not  that  this  action  is  confined  to  such 
areas,  for  it  may  be  observed  almost  everywhere,  and 
more  particularly  in  mountain-regions.  Outside  of 
deserts,  however,  the  wind  acts  chiefly  as  a  transporter 
of  rock-material.  In  all  latitudes  incoherent  deposits 
of  sand,  exposed  and  dried,  come  under  its  power, 
and  tend  to  be  piled  up  in  heaps  and  ridges  or  spread 
out  in  sheets.  In  this  way  certain  more  or  less 
prominent  land-features  owe  their  origin  directly  to 
wind ;  and  as  we  have  devoted  some  space  to  the 
consideration  of  the  action  of  ice 'as  a  special  agent 
of  erosion  and  accumulation,  we  shall  now  take  a 
rapid  glance  at  the  more  notable  surface-features  that 
result  from  the  destructive  and  reproductive  action  of 
the  atmosphere. 

In  desiccated  regions  rock-disintegration  and  the 
transport  and  accumulation  of  superficial  materials 
are  mainly  the  work  of  insolation  and  deflation — rain 
and  running  water  necessarily  play  a  very  subordinate 
part.  This  is  certainly  the  case  in  the  Sahara — the 
most  extensive  tract  of  desert  in  the  Old  World. 
This  vast  region  stretches  across  Africa  from  the  At- 
lantic coast  to  the  valley  of  the  Nile,  and  from  the 
northern  borders  of  the  Soudan  to  the  Atlas  Mount- 


25  2  EAR  TH  SCULP  TURE 

ains  and  the  Mediterranean,  an  area  equal  in  size  to 
two-thirds  of  Europe.  The  surface  of  the  Sahara  is 
sufficiently  diversified,  and  is  not,  as  popularly  sup- 
posed, entirely  covered  with  blowing  sand.  Dunes, 
no  doubt,  spread  over  enormous  territories,  but  wide 
tracts  and  broad  basins  of  loam  and  clay,  with  saline 
lakes  and  marshes,  likewise  present  themselves,  whilst 
elsewhere  rocky  and  stony  plateaux,  and  even  lofty 
mountains,  occupy  extensive  areas.  Ever  and  anon, 
moreover,  verdant  oases  appear,  and  these  are  so 
numerous  that  they  must  altogether  form  no  incon- 
siderable portion  of  the  whole  Sahara.  The  entire 
area  might  be  described  as  an  old  plateau  of  accumu- 
lation, built  up,  as  it  appears  to  be  for  the  most  part, 
of  horizontally  or  gently  inclined  strata.  Probably  its 
mean  altitude  is  not  less  than  2000  feet,  only  a  small 
portion  lying  to  the  south  of  Algeria  being  below  the 
level  of  the  Mediterranean.  The  rocky  areas  of  the 
region  are  broken  up  into  a  succession  of  narrower 
and  broader  terraces  or  plateaux — now  in  many  places 
traversed  by  dry,  winding  gullies,  ravines,  valleys,  and 
other  abandoned  watercourses,  or  largely  replaced  by 
groups  of  bare  pyramidal  hills,  buttes,  mesas,  and  ir- 
regular rock-masses.  Over  wide  areas  blowing  sands 
are  absent,  while  elsewhere  they  are  heaped  up  and 
spread  out  to  such  an  extent  that  the  rocky  frame- 
work of  the  country  becomes  entirely  concealed. 

Wind  erosion  is  naturally  best  studied  in  the  bare 
portions  of  the  desert.  Under  the  influence  of  inso- 
lation the  rocks  crumble  down,  and  the  disintegrated 


AEOLIAN  ACTION  253 

material  is  swept  onward  by  the  wind.  Hard,  com- 
pact stones  acquire  a  polish  like  that  given  by  a  lapid- 
ary's wheel,  while  rocks  of  unequal  consistency  yield 
irregularly,  the  softer  portions  being  removed  and 
the  harder  parts  left  standing  in  relief.  Where  the 
surface  of  the  land  is  very  uneven  the  air-currents 
streaming  between  opposing  heights  have  ground  out 
deep  hollows  and  gullies.  In  like  manner  curious 
niches,  cirques,  and  amphitheatres  have  been  excava- 
ted in  the  walls  of  the  dry  wadies.  Everywhere,  in- 
deed, the  rocks  are  abraded,  fretted,  honeycombed, 
and  undermined.  Undermining  is,  in  truth,  one  of 
the  most  notable  stages  in  the  general  reduction  of 
the  surface.  The  bulk  of  the  sand  driven  forward 
by  the  wind  rises  only  a  few  feet  above  the  surface, 
hence  cliffs  and  stacks  wear  away  rapidly  below  until 
the  overhanging  mass  collapses  and  topples  down, 
whereupon  the  same  action  is  repeated  upon  the  fallen 
debris.  Hence  isolated  rock-masses  often  take  peculiar 
mushroom-shapes. 

Among  the  most  fantastic  forms  assumed  under  the 
action  of  the  wind  are  those  met  with  among  granites 
and  granitoid  rocks.  Often  rising  boldly  above  the 
general  level,  they  show  no  trace  of  talus  or  debris, 
but  are  swept  bare  to  the  base,  and  to  the  fanciful 
Arab  they  often  simulate  the  appearance  of  ele- 
phants, apes,  camels,  panthers,  and  the  like.  In 
Europe  granite  hills  and  mountains  frequently  show 
rounded  summits,  and  are  usually  well  mantled  with 
talus.  In  the  desert,  on  the  other  hand,  they  are 


254 


EARTH  SCULPTURE 


much  more  rugged  and  abrupt,  their  precipitous 
flanks  bare  of  ddbris,  and  their  serrated  crests  and 
peaks  recalling,  according  to  Walther,  the  bold  and 
abrupt  dolomite  mountains  of  South  Tyrol.  The 
horizontally  arranged  strata  of  the  desert  assume  very 


FIG.  79.      WIND  EROSION:  TABLE  MOUNTAINS,  ETC.,  OF  THE  SAHARA 
(Mission  de  Chadames). 

different  forms,  and  have  been  carved  into  tabular, 
conical,  and  pyramidal  hills,  with  a  general  resem- 
blance to  the  buttes,  mesas,  and  pyramids  of  the 
Colorado  region.  (Fig.  79.)  When  the  strata  are 


FIG.  80.    WIND  EROSION  :  HARDER  BEDS  AMONGST  INCLINED  CRETACEOUS 
STRATA.  LIBYAN  DESERT.    (J.  Walther.) 

inclined  the  outcrops  of  the  harder  beds  project,  and 
we  have  in  like  manner  a  reproduction  of  the  escarp- 
ments and  dip-slopes  which  the  same  geological  struct- 
ure gives  rise  to  in  well  watered  lands.  (Fig.  80.) 


AEOLIAN  ACTION 


*55 


The  projecting  ledges  and  escarpments,  however,  are 
always  honeycombed  and  dressed  in  a  different  way, 
betokening  everywhere  the  characteristic  action  of 
the  wind. 

The  final  result  of  wind  erosion  is  the  reduction  of 
inequalities  and  the  production  of  a  plain-like  surface. 


ILL 


FIG.  81.    WIND  EROSION  :  STAGES  IN  THE  EROSION  AND  REDUCTION  OF  A 
TABLE-MOUNTAIN.    (J.  Walther.) 

In  the  Eastern  Sahara  wide  areas  of  rocky  land  have 
been  thus  levelled.  (Fig.  81.)  Such  areas  are  usually 
more  or  less  abundantly  besprinkled  and  paved  with 
angular  stones,  usually  dark  brown  or  black,  and  so 
highly  polished  that  they  glance  and  glitter  in  the  sun. 


256  EARTH  SCULPTURE 

It  is  obvious  that  such  stones  are  derivative  ;  they  are 
the  relics  of  massive  beds  of  sandstone,  through  which 
they  were  formerly  distributed,  and  which  have  since 
been  gradually  disintegrated  and  removed.  In  some 
places,  indeed,  massive  inclusions  of  the  kind  (man- 
ganese concretions),  of  all  shapes  and  sizes,  project 
from  the  surface  of  the  sandstone  in  which  they  are 
still  partly  embedded.  On  the  lee  side  of  such  con- 
cretions the  sandstone  has  been  sheltered  from  the 
attack  of  the  wind,  while  it  has  been  planed  away  in 


FIG.    82.      MANGANESE   CONCRETIONS   WEATHERED    OUT   OF  SANDSTONE  ; 
ARABAH  MOUNTAINS,  SINAI  PENINSULA.   (J.  Walther.) 

front.  No  stone  withstands  the  action  of  the  wind 
so  well  as  the  hard  flints,  jaspers,  and  silicious  con- 
cretions, which  are  so  commonly  met  with  in  the  sedi- 
mentary strata  of  the  Libyan  desert.  When  the  latter 
have  become  disintegrated  and  gradually  removed  by 
the  wind,  the  hard  nodules  and  concretions  remain, 
and  thus  the  broad  plains  are  covered  over  with  sheets 
of  "gravel"  and  shingle.  The  Sserir,  according  to 
Walther,  are  nothing  more  than  rocky  lands  levelled 
by  wind-erosion  ;  the  more  yielding  materials  have 


AEOLIAN  ACTION  257 

been  swept  away,  while  the  hard  inclusions  left  behind 
are  now  concentrated  at  the  surface. 

Another  result  of  deflation  may  be  referred  to. 
Now  and  again  in  wind-swept  plains  and  plateaux 
the  rocks,  according  to  their  nature,  are  variously 
affected.  Some  are  disintegrated  and  rotted  more 
readily  than  others.  These,  therefore,  tend  to  be 
more  rapidly  reduced  below  the  general  level,  and 
shallow  basins  are  thus  formed  which  are  sometimes 
occupied  by  water  for  shorter  or  longer  intervals. 
Such  is  probably  the  origin  of  the  Caldeiraos  of 
Bahia,  where  the  general  configuration  of  the  surface 
has  some  resemblance  to  that  of  an  ice-worn  region— 
gently  rolling  ground,  namely,  showing  innumerable 
shallow  depressions  winding  amongst  multitudinous 
bare-backed,  dome-shaped  rocks. 

The  disintegrated  material  removed  from  a  rocky 
desert  is  eventually  spread  out  and  piled  up  in  sheets 
and  heaps  of  sand,  which  travel  onwards  in  the  direc- 
tion of  the  prevalent  wind.  In  the  Eastern  Sahara 
bare  rocky  plateaux  prevail,  and  sand-wastes  are 
usually  of  inconsiderable  extent.  In  the  Western 
Sahara,  on  the  other  hand,  the  whole  area  is  more 
or  less  smothered  in  sand.  There  vast  stretches  of 
dunes  move  with  the  trade-winds.  Advancing  to 
the  south-west,  they  reach  the  banks  of  the  Niger 
and  the  Senegal,  and  are  here  and  there  forcing  these 
rivers  southward.  Again,  passing  to  the  west,  they 
touch  the  Atlantic  coast  between  Cape  Bojador  and 
Cape  Blanco,  and  stream  out  to  sea  so  as  to  form 


258  EARTH  SCULPTURE 

a  belt  of  sand-banks  extending  several  miles  from 
the  shore.  For  long  ages,  therefore,  a  great  current 
of  sand  has  been  constantly  flowing  out  of  the  desert. 

The  dunes  of  a  desert  appear  to  move  more  readily 
than  those  of  maritime  regions.  Possibly  this  may 
be  due  to  the  better  rolled  character  of  the  constitu- 
ent grains,  to  the  drier  condition  of  the  sand,  to  the 
want  of  any  binding  materials,  and  the  absence  of 
a  fixed  nucleus,  such  as  is  so  commonly  acquired  for 
the  formation  of  coastal  dunes.  In  the  central  por- 
tions of  a  desert  they  are  generally  arranged  in  series 
of  long  parallel  undulations,  that  extend  in  a  direc- 
tion at  right  angles  to  that  of  the  prevalent  wind. 
Elsewhere  they  may  be  more  irregular  in  their 
grouping  and  arrangement,  individual  sand  hills  not 
infrequently  assuming  a  crescentic  or  sickle-like 
shape.  They  vary  much  in  height,  not  often  exceed- 
ing 250  feet,  although  occasionally  reaching  500  or 
even  600  feet. 

It  need  hardly  be  said  that  dunes  are  not  restricted 
to  desert  regions.  Wherever  incoherent  deposits 
are  dried  and  exposed  to  the  air,  they  are  liable  to 
drift  with  the  wind.  Hence  blowing  sands  are  well 
developed  upon  certain  sea-coasts  and  lake  shores, 
and  in  the  broad,  flat  valleys  of  many  large  rivers. 
If  the  surface  over  which  sand  is  blown  be  level  and 
free  from  obstructions,  the  sand  does  not  necessarily 
accumulate  in  heaps  and  banks,  but  is  often  spread 
out  in  successive  horizontal  layers,  forming  a  sand- 
plain.  But  wherever  obstructions  intervene,  such  as 


AEOLIAN  ACTION 


259 


prominent  rocks,  trees,  bushes,  or  what  not,  these 
give  rise  to  inequalities  in  the  distribution  of  the 
sand.  A  steep  talus  of  grains  gathers  in  the  sheltered 
lee,  while  a  more  gently  sloping  bank  gradually  rises 
on  the  windward  side  of  the  obstruction,  until  this  is. 


FIG.  83.     FORMATION  OF  SAND-DUNES. 

o,  obstacle  intercepting  sand  ;  «/,  windward  side  ;  /,  lee  side;  jj,  sea-level. 

eventually  overtopped  and  buried.  (See  Fig.  83.)  In 
this  way  a  dune  is  formed,  and  continues  to  increase 
until  it  reaches  its  maximum  height,  determined  by 
the  strength  of  the  wind  and  the  supply  of  the 
materials,  and  probably  in  some  measure  also  by  the 


FIG.  84.     ADVANCE  OF  SAND-DUNES, 

Illustrated  by  the  burial  of  a  church,  and  its  subsequent  reappearance,  in  the  neighbourhood 
of  the  Kurisches  Haff.     (G.  Berendt.) 

size  of  the  sand-grains.  As  the  sand  continually 
travels  up  the  gentler  windward  slope,  and  comes 
to  rest  on  the  steeper  leeward  slope,  it  follows  that 
a  dune  itself  must  constantly,  if  slowly,  move  for- 
wards. Thus  in  time  the  nucleus  that  gave  origin 


260  EARTH  SCULPTURE 

to  such  a  sand-hill  may  become  again  exposed.  (See 
Fig.  84,  p.  259.) 

Coastal  sand-hills,  like  those  of  inland  regions,  are 
frequently  arranged  in  successive  parallel  ridges  or 
undulations.  These,  however,  are  often  interrupted 
by  transverse  hollows,  and  the  dunes  frequently  run 
into  one  another  irregularly.  I  n  other  places  little  or  no 
parallel  arrangement  can  be  traced,  the  hills  and  hum- 
mocks showing  a  tumultuous  and  tumbled  surface  of 
winding  and  straggling  ridges,  of  isolated  banks  and 
knolls,  and  confused  groups  of  mounds  and  hillocks, 
the  hollows  amongst  which  form  a  perfect  labyrinth. 
Should  grasses  or  other  vegetation  clothe  the  dunes> 
these  become  fixed,  but  in  the  absence  of  any  plant- 
growth  the  surface  of  the  sand-hills  is  kept  in  constant 
motion  by  the  wind. 

In  the  hollows  amongst  sand-dunes  marshes,  pools, 
and  lakes  now  and  again  appear.  In  some  parts  of 
the  Sahara,  for  example,  long  straggling  basins  of 
groundwater  extend  between  the  sand-ridges.  Again, 
the  advance  of  sand-dunes  from  a  coast  has  often  ob- 
structed the  natural  drainage,  and  formed  swamps 
and  lakes  of  larger  or  smaller  extent.  The  lagoons, 
which  in  many  places  are  separated  from  the  sea, 
have  frequently  been  cut  off  from  the  outside  ocean 
by  the  combined  action  of  the  waves  and  the  wind  in 
raising  up  sand-banks  and  -dunes. 

In  desert  regions  the  bulk  of  the  sand  driven  for- 
ward by  the  wind  rises  to  no  great  height  above  the 
surface  ;  its  abrading  and  scouring  action  is  largely 


AEOLIAN  ACTION  261 

confined  to  the  basal  portions  of  the  rocks  against 
which  it  is  borne.  But  the  finer-grained  matter — the 
powdery  dust — is  often  swept  upwards  to  great 
heights,  and  may  be  transported  for  hundreds  or 
even  thousands  of  miles  from  the  place  of  its  origin. 
As  might  have  been  expected,  however,  it  is  over  the 
region  immediately  surrounding  a  desiccated  area 
that  the  dust  chiefly  falls.  In  time  such  regions  be- 
come more  or  less  thickly  mantled  with  this  dust, 
which  usually  yields  a  fertile  soil.  After  long  ages  of 
accumulation  the  whole  surface  of  the  dust-covered 
tracts  becomes  greatly  modified.  Inequalities  are 
smoothed  over,  and  everywhere  softly  flowing  feat- 
ures are  produced.  As  no  hard-and-fast  line  separates 
an  area  of  wind-erosion  from  one  of  dust-accumula- 
tion, sand  and  dust  become  commingled  along  the 
borders  of  the  two  regions,  or  there  is  a  gradual  transi- 
tion from  the  one  kind  of  material  to  the  other.  The 
fertility  of  the  Nile  Valley  is  rightly  attributed  to  the 
fine  silt  and  loam  of  the  annual  floods,  but  desert- 
dust  has  also  added  its  share  to  the  soil  of  Egypt. 
Similarly  it  is  believed  that  dust  has  played  an  im- 
portant part  in  the  formation  of  the  fine  porous  soils 
of  many  other  lands.  According  to  Baron  Richtho- 
fen,  the  vast  loss  accumulations  of  China  are  true 
dust-deposits.  Loss  is  a  fine-grained,  homogeneous 
calcareous  and  sandy  loam,  penetrated  vertically  by 
numerous  root-like  pores  and  tubes,  which  have  the 
same  effect  on  the  deposits  as  joints  in  rocks — they 
allow  the  loss  to  cleave  in  a  vertical  direction.  When 


262  EARTH  SCULPTURE 

it  is  intersected,  therefore,  by  streams  and  rivers  it 
forms  bold  bluffs  and  cliffs.  It  usually  contains  land- 
shells,  and  now  and  again  the  bones  of  land  animals. 
Fresh-water  shells  rarely  occur,  while  marine  organ- 
isms are  wholly  wanting.  In  Northern  China  this 
remarkable  accumulation  covers  vast  areas,  and  at- 
tains in  places  a  thickness  of  1500  feet  or  even  of 
2000  feet.  The  regions  occupied  by  it  have  the 
aspect  of  extensive  plains,  which  look  as  if  they 
might  be  traversed  with  ease  in  any  direction.  They 
are  abundantly  intersected,  however,  by  deep  valleys 
and  precipitous  rock-like  gullies  and  ravines,  in  the 
vertical  walls  of  which  the  natives  have  excavated 
their  dwelling-places.  Richthofen  believes  that  this 
great  deposit  has  been  gradually  accumulated  by  the 
winds  flowing  outwards  from  the  desiccated  regions 
of  Central  Asia.  Vast  quantities  of  fine  sand  and 
dust  are  there  swept  up  during  storms  and  scattered 
far  and  wide,  and  in  this  manner  adjoining  territories, 
such  as  the  grassy  steppes,  are  ever  and  anon  receiv- 
ing increments  to  their  soil.  The  finely  sifted  mate- 
rial thus  obtained  is  highly  fertile  and  favours  the 
growth  of  the  grasses,  so  that  every  fresh  deposit  of 
dust  tends  to  become  fixed,  and  the  steppe-formation 
continues  to  increase  in  thickness.  It  is  this  con- 
tinued growth  of  vegetation,  keeping  pace,  as  it  were, 
with  the  periodical  accumulation  of  soil,  which  is  sup- 
posed to  produce  the  porous  capillary  structure  re- 
ferred to  above  as  the  cause  of  the  vertical  cleavage 
of  the  loss. 


AEOLIAN  ACTION  263 

Loss  occurs  in  many  other  countries,  but  it  no- 
where attains  so  vast  a  development  as  in  China.  In 
Europe  we  meet  with  it  in  the  valley  of  the  Rhine 
and  in  the  low  grounds  traversed  by  the  Danube, 
where,  although  it  forms  no  enormous  plains  like 
those  of  Northern  China,  it  nevertheless  mantles  the 
ground  so  as  in  some  measure  to  conceal  the  older 
features  of  the  land.  The  extensive  sheets  of  black 
earth  which  cover  the  surface  of  the  great  plains  of 
Southern  Russia  are  also  a  variety  of  loss.  The 
origin  of  the  European  deposits  has  been  much  dis- 
cussed by  geologists,  but  it  seems  to  be  now  the  gen- 
eral opinion  that  the  materials  of  the  loss  were,  in  the 
first  place,  introduced  into  the  low  grounds  chiefly  by 
the  flooded  rivers  and  inundations  of  the  Ice  Age. 
Muddy  water  escaping  from  the  glaciers  of  the  Alps 
and  other  mountains,  and  from  the  terminal  front  of 
the  great  " inland  ice"  of  Northern  Europe,  doubtless 
drowned  wide  areas,  while  torrents  derived  from  the 
melting  snows  of  extraglacial  tracts  must  likewise 
have  swept  down  large  quantities  of  fine-grained  sedi- 
ment. Thus,  long  after  the  periodical  inundations  of 
glacial  times  had  diminished  in  extent  and  finally 
ceased,  the  lower  reaches  of  the  great  valleys  and  the 
broad  plains,  formerly  subject  to  floods,  must  have 
been  more  or  less  sheeted  with  sandy  loams.  We 
know  now  that  Tundra-  and  Steppe-conditions  have 
succeeded  in  Central  Europe.  Already  towards  the 
close  of  glacial  times  a  well  marked  Tundra-fauna 
had  spread  south  to  the  Alps  and  west  into  France 


264  EARTH  SCULPTURE 

and  England.  At  that  period  the  climatic  conditions 
were  probably  such  as  are  now  experienced  in  North- 
ern Siberia.  Eventually,  however,  these  conditions 
gradually  gave  way, — the  Tundra-fauna  began  to  re- 
treat, until  by  and  by  it  was  supplemented  by  a  no 
less  characteristic  Steppe-fauna,  the  range  of  which 
seems  to  have  been  as  extensive  as  that  of  the  former. 
The  Arctic  lemming,-  Arctic  fox,  reindeer,  musk-ox, 
and  glutton  of  the  Tundras  were  now  replaced  by  the 
jerboa,  pouched  marmot,  tailless  hare,  little  hamster 
rat,  and  other  forms,  the  common  denizens  to-day  of 
the  Steppes  of  Eastern  Russia  and  Western  Siberia. 
It  is  certain,  then,  that  a  dry  Steppe-climate  has  pre- 
vailed at  no  distant  date,  geologically  speaking, 
throughout  Central  and  Western  Europe.  Thus  we 
may  be  sure  that  dust-storms  must  formerly  have 
been  as  common  in  France  and  Belgium  and  the  re- 
gions lying  to  the  east  as  they  are  now  in  Russian 
and  Asiatic  Steppes.  It  was  during  the  prevalence 
of  such  climatic  conditions,  as  geologists  think,  that 
the  wide-spread  flood-loams  of  the  Glacial  Period  were 
so  largely  re-assorted  and  remodified  by  deflation,  and 
the  lossic  accumulations  assumed  their  present  aspect 
and  distribution. 

Mention  has  been  made  of  the  fact  that  marshes 
and  lakes  occur  now  and  again  in  the  hollows  amongst 
sand-dunes.  They  are  met  with  likewise  amongst 
dust-deposits.  Thus  pools  and  large  and  small  sheets 
of  water  sometimes  dapple  the  surface  or  extend  over 
broad  areas  of  the  wind-swept  Steppes.  Such  basins, 


AEOLIAN  ACTION  265 

doubtless,  are  partly  due  to  the  unequal  distribution  or 
heaping-up  of  fine  sand  and  dust.  In  some  cases, 
however,  they  seem  to  have  been  caused  by  the  un- 
equal removal  of  superficial  materials. 

In  fine,  then,  we  conclude  that  wind-erosion  is  most 
effective  in  dry,  desert  regions.  Its  influence  is,  no 
doubt,  world-wide  ;  but  as  an  active  agent  in  levelling 
the  land — in  cutting,  carving,  undermining,  and  re- 
moving rock — wind  plays  the  dominant  part  in  de- 
siccated lands.  We  note,  further,  that  the  forms 
assumed  by  rocks  subject  to  wind-erosion  are  largely 
determined  by  geological  structure  and  the  nature  of 
the  rocks  themselves,  just  as  in  temperate  latitudes 
feeble  structures  and  relatively  soft  rocks  are  the  first 
to  yield.  Lastly,  we  recognise  that  certain  wind- 
blown accumulations  have  a  world-wide  distribution, 
and  occur  under  all  conditions  of  climate.  Sand-dunes 
may  be  met  with  wherever  incoherent  deposits  of 
sufficiently  fine  grain  are  exposed  to  the  action  of 
the  wind.  Dust,  on  the  other  hand,  is  pre-eminently 
a  product  of  relatively  dry  regions  and  of  deserts — 
wherever,  indeed,  the  land  is  naked  or  only  partially 
clothed  with  vegetation,  dust  is  formed,  and  may  be 
swept  up  and  transported  by  the  wind. 


CHAPTER  XIII 

LAND-FORMS  MODIFIED  BY  THE  ACTION  OF 
UNDERGROUND  WATER 

DISSOLUTION    OF    ROCKS UNDERGROUND    WATER-ACTION    IN  CAL- 
CAREOUS LANDS KARST-REGIONS   OF   CARINTHIA  AND  ILLYRIA 

E7FECTS    OF    SUPERFICIAL     AND    SUBTERRANEAN     EROSION 

TEMPORARY      LAKES CAVES      IN      LIMESTONE CAVES     IN    AND 

UNDERNEATH    LAVA — "CRYSTAL    CELLARS" — ROCK-SHELTERS 
SEA-CAVES. 

IN  Chapter  VII.  it  was  pointed  out  that  subterranean 
action  had  played  a  most  important  part  in  the 
production  of  certain  surface-features.  In  particular 
it  was  shown  that  depression  of  the  surface  has  fre- 
quently taken  place  as  a  result  of  that  action.  We 
have  now  to  consider  another  kind  of  action  altogether, 
which,  although  by  no  means  so  important  as  that 
just  referred  to,  nevertheless  now  and  again  causes 
the  surface  in  certain  regions  to  subside.  Rocks,  as 
we  have  seen,  are  very  variously  acted  upon  by  water 
—a  few  are  readily  soluble,  but  the  great  majority 
are  not.  The  most  important  of  the  soluble  rocks 
are  rock  salt,  gypsum,  and  limestones  of  every  kind. 
These  are  all  more  or  less  easily  removed  by  meteoric 
water.  Rock  salt  is  so  very  soluble  that  it  is  seldom 

266 


ACTION  OF  UNDERGROUND   WATER         267 

or  never  found  cropping  out  at  the  surface  ;  any  sur- 
face-exposure in  temperate  lands  would  rapidly  disap- 
pear. It  is  only  in  dry  and  rainless  tracts,  therefore, 
that  rock  salt  can  exist  as  a  superficial  accumulation. 
Gypsum  is  more  readily  dissolved  than  limestone,  but 
both  rocks  become  eaten  into  at  the  surface,  and,  ac- 
cording to  circumstances,  are  more  or  less  rapidly 
washed  away.  This  process  of  dissolution,  it  is  need- 
less to  add,  is  not  confined  to  the  surface.  Meteoric 
water  penetrates  the  ground,  and  circulates  through 
the  crust  to  considerable  depths.  After  pursuing  a 
shorter  or  longer  course,  it  reappears  at  the  surface 
as  springs,  the  waters  of  which  are  more  or  less 
abundantly  charged  with  dissolved  mineral  matter, 
according  to  the  nature  of  the  rocks  through  which  it 
has  passed.  In  this  way  enormous  quantities  of  sol- 
uble materials  are  brought  up  from  below  ;  in  short, 
wholesale  chemical  erosion  goes  on  underground. 
It  follows  that  in  regions  where  soluble  rocks  enter 
largely  into  the  framework  of  the  land  the  surface 
must  in  time  subside  slowly  or  suddenly.  The  copious 
outpouring  of  brine-springs  gradually  reduces  beds 
and  sheets  of  rock  salt,  and  the  overlying  strata  sink 
down  and  thus  produce  depression  at  the  surface. 
And  the  same  result  is  brought  about  by  the  dissolu- 
tion of  gypsum,  limestone,  and  dolomite.  Sometimes 
the  surface  slowly  subsides,  but  now  and  again  it  col- 
lapses suddenly,  producing  earthquakes,  accompanied 
by  much  fracturing  and  shifting  of  the  rocks.  Thus 
it  is  believed  that  the  earthquakes  which  disturbed 


268  EARTH  SCULPTURE 

the  Visp-Thal  in  Valais  during  the  summer  and  au- 
tumn of  1855  were  the  result  of  the  caving-in  of  the 
rocks  consequent  on  the  dissolution  and  removal  of 
gypsum,  for  the  springs  of  that  district  bring  to  the 
surface  annually  over  200  cubic  metres  of  the  mineral 
in  solution.  Similarly,  it  can  hardly  be  doubted  that 
many  of  the  larger  and  deeper  depressions  of  the  sur- 
face which  appear  in  regions  of  calcareous  rock  are 
the  result  of  sudden  collapse  due  to  the  removal  of 
material  by  underground  water. 

As  rock  salt  and  gypsum  do  not  enter  largely  into 
the  composition  of  the  crust,  they  are  less  important 
from  our  point  of  view  than  limestones.  The  latter 
not  only  attain  in  many  cases  a  much  greater  thick- 
ness, but  they  are  far  more  widely  distributed,  and 
extend  over  much  broader  areas  of  the  earth's  sur- 
face. It  is  in  regions  of  calcareous  rocks,  therefore, 
where  underground  water  plays  the  most  prominent 
rdle,  and  where  its  action  in  modifying  surface-features 
is  best  displayed.  In  a  former  chapter  reference  has 
been  made  to  the  fact  that  in  countries  occupied  by 
limestone,  the  drainage  is  often  largely  or  even  wholly 
conducted  underground.  The  rocks  are  so  penetrated 
in  all  directions  by  rifts,  clefts,  and  tunnels,  that  the 
water  which  falls  at  the  surface  very  soon  disappears. 
Concerning  the  origin  of  these  subterranean  spaces 
there  is  not  much  difference  of  opinion.  Geologists 
recognise  that  they  have  been  worked  out  by 
chemical  and  mechanical  water-erosion.  But  while 
some  have  maintained  that  the  underground  water 


ACTION  OF  UNDERGROUND   WATER         269 

has  licked  and  worn  out  a  passage  for  itself  chiefly 
along  the  normal  divisions  of  the  rocks — their  joints 
and  bedding-planes — others  have  held  that  the  main 
lines  of  underground  drainage  have  been  determined 
by  faults  or  dislocations.  Both  views  are  doubtless 
true  :  some  caves  and  underground  tunnels  appear  to 
have  no  connection  with  faults  ;  others,  on  the  con- 
trary, follow  these,  although  many  of  the  channels 
connected  with  them  have  been  worked  out  along 
joints  and  bedding-planes. 

Underground  water  usually  follows  a  zigzag  and 
irregular  course — now  plunging  downwards  at  high 
angles,  or  even  vertically,  through  relatively  con- 
stricted clefts  and  fissures  ;  now  winding  through  ap- 
proximately horizontal  tunnels,  or  forming  lake-like 
expansions  in  broad  and  lofty  halls  and  chambers ; 
now  dividing  into  more  or  less  numerous  torrents  and 
streams,  which  zigzag  downwards  to  lower  and  lower 
levels.  In  time  many  changes  are  effected.  Here 
and  there  passages  are  blocked  with  sediment  or  by 
falls  from  the  roof,  and  become  partially  or  wholly 
abandoned,  the  water,  dammed  back,  rising  and  mak- 
ing its  escape  by  other  clefts  and  hollows.  Thus 
eventually  the  limestone  becomes  traversed  in  all 
directions  by  a  perfect  net-work  of  intercrossing  chan- 
nels— winding  and  angulate,  low  and  lofty,  broad  and 
narrow — many  of  which  become  abandoned  by  the 
water  as  it  works  its  way  to  lower  and  lower  lev- 
els. To  what  depth  from  the  surface  considerable 
tunnels  can  be  excavated  by  chemical  and  mechanical 


270  EARTH  SCULPTURE 

erosion  we  cannot  tell.  It  is  obvious,  however,  that 
a  limit  must  be  reached  when  the  pressure  of  the 
superincumbent  and  surrounding  rocks  becomes  so 
great  that  no  vacant  spaces  can  exist.  Water  de- 
scending from  the  surface  must  thus  eventually  be 
forced  by  hydrostatic  pressure  to  rise  again  and 
escape  at  lower  levels  than  its  source.  Large  under- 
ground channels,  therefore,  probably  descend  to  no 
great  depth  from  the  surface,  and  their  size  is  natur- 
ally limited  by  the  structure  of  the  rock  in  which  they 
are  excavated.  Where  this  is  much  jointed  and  fis- 
sured it  is  obvious  that  the  span  of  a  cavern  cannot 
be  great ;  the  disjointed  rocks,  losing  support,  tend  to 
collapse.  The  widest  underground  chambers  do  not 
exceed  100  yards  in  width. 

In  course  of  time  the  whole  surface  of  a  country  is 
gradually  lowered  by  denudation.  This  change  goes 
on  most  rapidly  no  doubt  in  regions  where  the  super- 
ficial rocks  are  more  or  less  impermeable.  But  lands 
composed  chiefly  of  limestone  do  not  escape — corro- 
sion, especially,  proceeds  more  or  less  rapidly.  Ever 
and  anon,  too,  the  surface  sinks  slowly  or  suddenly  as 
the  case  may  be,  consequent  on  the  withdrawal  of 
rock-material  from  below.  The  peculiar  deformations 
caused  by  such  changes  are  among  the  most  charac- 
teristic features  of  limestone  regions.  Typical  regions 
of  the  kind  show  no  regular  river-systems ;  brooks 
and  rivulets  are  wanting.  Water  sinks  at  once  into 
the  ground  by  pipes  and  swallow-holes,  clefts  and 
fissures.  In  the  lower-lying  parts  of  such  lands  now 


ACTION  OF  UNDERGROUND   WATER        271 

and  again  rivers  suddenly  emerge  at  the  surface,  and 
after  usually  a  short  course  may  again  disappear  be- 
low ground.  In  the  rainy  season  water  often  rises 
through  the  apertures  by  which  the  surface  is  more 
or  less  abundantly  pierced,  and  dry  valleys  and  wide 
basin-shaped  depressions  become  flooded.  Of  course 
when  the  supply  fails  the  water  again  returns  to  the 
depths  from  which  it  was  discharged. 

In  the  karst-regions  of  Carinthia  and  Illyria  these 
phenomena  are  very  well  displayed.  The  funnel- 
shaped  depressions  communicating  with  underground 
galleries,  which  with  us  are  termed  swallow-holes,  are 
known  in  Carinthia  as  dolinas.  These  vary  in  width 
and  depth  from  a  few  yards  up  to  half  a  mile  in  width, 
and  from  100  to  200  yards  and  more  in  depth.  Most 
of  them,  however,  are  small — 40  or  50  yards  across, 
and  about  30  yards  or  so  in  depth.  Their  bottom  is 
somewhat  flat,  and  often  covered  with  loam  or  clay. 
The  larger  ones  are  relatively  shallower  in  proportion 
to  their  width  than  the  others.  Not  less  character- 
istic features  of  the  karst-lands  are  the  so-called  blind- 
valleys  and  dry-valleys.  Through  the  former  a  river 
flows  to  disappear  into  a  tunnel  at  the  closed  or  blind 
end.  The  dry-valleys  have  no  river ;  the  bottom  is 
usually  irregular  and  often  pitted  with  dolinas.  Be- 
sides these  land-forms,  geographers  recognise  another 
kind  of  depression,  the  so-called  "kettle-valleys," 
which  are  trough-like  or  dish-shaped  basins  of  vari- 
able extent,  some  of  them  having  an  area  of  several 
hundred  square  miles.  Not  infrequently  the  smaller 


272  EARTH  SCULPTURE 

ones  run  in  parallel  zones  following  the  direction  of 
the  strike  of  the  strata.  All  these  surface-features  are 
for  the  most  part  the  result  of  underground  erosion. 
Some  of  the  dolinas  may  have  been  eroded  by  water 
descending  through  fissures  from  the  surface  ;  but. 
probably  the  greater  number,  and  certainly  all  the 
larger  ones,  have  been  caused  by  the  caving-in  of 
underground  tunnels.  So,  again,  the  blind-valleys  and 
dry-valleys  appear  in  most  cases  to  form  part  of  the 
subterranean  drainage-system,  now  exposed  by  col- 
lapse of  roof  and  the  general  degradation  of  the  sur- 
face. The  natural  bridges  or  arches  which  are  seen 
often  enough  in  such  regions  are  simply  the  relics  of 
old  underground  tunnels  and  waterways,  the  ruins  of 
which  often  cumber  the  depressions  of  the  surface. 
It  is  hardly  worth  while  adding  that  the  numerous 
limestone  caverns  in  which  geologists  have  hunted 
so  successfully  for  remains  of  primeval  man  and  his 
associates  are  merely  the  abandoned  courses  of  an- 
cient underground  streams  and  rivers.  Almost  ev- 
erywhere, indeed,  in  great  limestone-regions  one  may 
trace  at  the  surface  evidence  of  the  effects  produced 
by  subterranean  erosion.  The  trough-shaped  basins 
(kettle-valleys)  referred  to  above  seem  to  owe  their 
origin  in  the  first  place  to  determinate  fissures. 
These  are  widened  by  the  action  of  the  surface-water 
as  it  passes  underground,  and  the  depression  at  the 
surface  increases  as  the  rock  becomes  undermined, 
collapse  taking  place  from  time  to  time.  If  the  col- 
lapse be  recent  the  bottom  of  the  kettle-valley  is 


ACTION  OF  UNDERGROUND   WATER        273 

strewn  with  broken  rock-Mrzs.  Not  a  few  kettle- 
valleys  in  limestone-plateaux,  however,  may  have  been 
partially  excavated  by  superficial  water-action  before 
the  system  of  underground  drainage  was  established, 
but  by  the  action  of  the  latter  they  have  since  been 
more  or  less  modified.  It  may  be  taken  as  generally 
true  that  most  of  the  depressions  or  basins,  great  and 
small,  which  are  so  characteristic  of  karst-lands,  are 
either  largely  or  wholly  due  to  the  corrosive  and 
erosive  action  of  underground  water. 

Lakes,  as  we  have  seen,  often  appear  periodically 
in  these  regions.  Some  are  very  regular  in  their 
coming  and  going,  others  only  show  at  intervals  after 
unusually  heavy  rain  or  long-continued  wet  weather. 
One  of  the  best-known  examples  is  the  Lake  of  Jes- 
sero,  or  Zirknitz,  in  Carniola,  which  appears  now  and 
then  in  the- broad  valley  of  the  Planina.  This  river, 
after  flowing  underground  for  a  long  distance,  returns 
to  the  surface,  and  shortly  afterwards  winds  through 
a  wide  plain  encircled  by  high  cliffs  of  limestone. 
The  plain  is  pierced  by  hundreds  of  dolinas,  from 
which,  after  excessive  or  continuous  rain,  the  water 
wells  and  rushes  until  the  whole  wide  area  is  trans- 
formed into  a  lake.  The  extent  and  depth  and  the 
duration  of  this  temporary  lake  vary  ;  and  the  inter- 
vals between  its  successive  appearances  are  likewise 
inconstant ;  sometimes  only  a  year,  or  two  or  three 
years  may  elapse,  but  intervals  of  ten  and  even  of 
thirty  years  have  been  experienced.  Not  a  few  de- 
pressions in  the  surface  of  calcareous  tracts  may  be 


274  EARTH  SCULPTURE 

rendered  impermeable  by  the  accumulation  in  them  of 
loam  and  clay,  and  these  may  then  be  occupied  by 
permanent  lakes. 

The  influence  of  subterranean  water  is  not,  of 
course,  confined  to  regions  of  soluble  rocks.  Where- 
ever  water  circulates  in  the  crust  rocks  are  attacked, 
and  their  constituents  become  liable  to  chemical 
change.  In  this  manner  immense  quantities  of  min- 
eral matter  are  brought  up  from  below,  some  of  it  to 
be  thrown  down  at  the  surface,  where  in  time  it  may 
form  massive  accumulations.  The  mechanical  action 
of  subterranean  water  is  also  recognised  almost  every- 
where, and  more  particularly  in  places  where  the  geo- 
logical structure  is  weak,  where  rocks  are  in  a  state 
of  unstable  equilibrium.  But  the  effect  of  under- 
ground water  in  bringing  about  rock-falls  and  land- 
slips in  such  regions  has  already  been  sufficiently 
discussed. 

Although  caverns  naturally  occur  most  numerously 
and  attain  the  largest  size  in  the  more  readily  soluble 
rocks,  they  are  also  met  with  in  many  other  kinds. 
They  appear,  for  example,  not  infrequently  in  lava. 
Some  of  the  smaller  of  these  are  merely  large  blisters 
or  bubbles,  formed  by  the  segregation  of  the  absorbed 
water-vapour  while  the  lava  was  in  a  semi-fluid  condi- 
tion. The  more  extensive  lava-caves  have  a  different 
origin.  While  lava  is  flowing  it  necessarily  cools  rap- 
idly at  the  surface,  and  in  this  way  becomes  crusted 
over.  If  the  crust  thus  formed  be  of  sufficient  thick- 
ness and  strength,  it  remains  steadfast,  forming  a  kind 


ACTION  OF  UNDERGROUND    WATER         275 

of  tunnel,  out  of  which  the  still  liquid  lava  issues.  Such 
lava-caves  are  of  common  occurrence  in  Hawaii, 
Mexico,  California,  the  Canary  Islands,  Iceland,  etc. 
Some  are  only  a  few  feet  in  height  and  breadth,  others 
may  be  20  to  30  feet  broad,  6  to  10  feet  in  height,  and 
many  yards  in  length.  In  certain  volcanic  regions 
lava-caves  obtained  much  larger  dimensions,  but  there 
is  reason  to  believe  that  these  have  been  modified  by 
subsequent  erosion.  One  in  Hawaii  has  a  width  at 
the  entrance  of  130  feet,  a  height  of  20  feet,  and  a 
length  of  260  feet.  Another  (the  Raniaka  Cave)  is 
1 200  feet  long.  Water  flowing  in  cavities  under  the 
lava-coulees  of  Auvergne  (as  in  the  neighbourhood 
of  Clermont)  has  cut  out  courses  in  the  subjacent 
granite,  and  issues  at  the  lower  ends  of  the  lava- 
streams  through  natural  arcades.  And  many  similar 
examples  of  subterranean  tunnels  and  caves  might  be 
cited  from  other  regions,  where  the  erosion  has  been 
effected  chiefly  by  the  mechanical  action  of  water 
upon  relatively  insoluble  rocks. 

Mention  may  also  be  made  of  the  great  cavities 
which  occasionally  occur  in  faults.  The  spaces  be- 
tween the  two  walls  of  a  fault  or  dislocation  are 
usually  filled  up  either  with  rock-dS$rw,  or  subse- 
quently infiltrated  mineral  matter,  or  with  both. 
Now  and  again,  however,  the  filling-up  is  only  partial, 
and  chambers  of  some  size  remain.  These  are  often 
lined  with  finely  crystallised  minerals,  and  form  what 
are  known  in  Switzerland  as  "  crystal-cellars." 

Of  caves  solely  due  to  erosion  it  is  not  necessary  to 


276  EARTH  SCULPTURE 

say  much.  Shallow  caves  (rock-shelters)  are  fre- 
quently met  with  in  river-valleys,  where  one  can  see 
that  they  owe  their  origin  to  the  under-cutting  action 
of  the  water.  More  extensive  are  the  caves  often 
excavated  by  the  sea.  These  necessarily  vary  in 
appearance  with  the  character  of  the  rocks  in  which 
they  are  excavated.  The  presence  of  a  cave  indicates 
some  weak  structure — some  rock  or  rock-arrangement 
\vhich  has  offered  less  resistance  to  the  attack  of  waves 
and  breakers.  Vertical  dikes  of  basalt,  for  example, 
are  often  so  abundantly  jointed,  that  they  are  broken 
up  and  removed  more  readily  than  the  rocks  they 
traverse,  although  the  latter  may  consist  of  "softer" 
material,  such  as  sandstone.  The  highly  jointed 
basalt,  notwithstanding  its  superior  hardness,  is  easily 
shattered.  The  mere  force  of  the  waves  combined 
with  hydraulic  pressure  in  some  joints,  and  the  com- 
pression and  expansion  of  air  in  others,  suffices  to 
rupture  and  burst  the  weak  structure,  and  with  each 
drop  of  the  wave  large  and  small  fragments  may 
sometimes  be  seen  falling  from  the  roof  and  sides  of 
the  cave.  The  cave  thus  increases  in  height  as  the 
sea  works  its  way  inland,  until  not  infrequently  it 
communicates  with  the  surface  by  a  "  blow-hole," 
through  which  in  storms  not  only  spray  but  spouts  of 
water,  and  even  gravel  and  larger  stones,  are  ejected. 
Similar  caves  are  frequently  formed  in  well  jointed 
sandstones  and  in  many  other  kinds  of  rock.  They  are 
very  common,  for  instance,  in  Orkney  and  Shetland, 
and  they  are  well  known  also  in  Cornwall  and  the 


ACTION  OF  UNDERGROUND   WATER        277 

West  of  Ireland.  In  time  the  whole  roof  of  such 
caves  may  give  way,  and  the  latter  then  appear  as 
narrow  ravine-like  or  gorge-like  inlets.  This  can 
happen  only  when  the  land-surface  does  not  rise  to 
any  great  height  above  the  sea.  When  the  rocks 
above  a  sea-cave  are  too  strongly  built  or  too  thick 
to  permit  of  a  downfall  of  the  roof,  the  cave  may  at- 
tain very  considerable  dimensions.  But  as  all  rocks 
are  traversed  by  lines  of  weakness,  a  limit  must  be 
reached  beyond  which  caves  cannot  be  widened.  By 
and  by  the  rocks  will  cease  to  be  self-supporting,  and 
collapse  must  take  place. 

Caves  of  marine  origin  are  seldom  met  with  far 
removed  from  existing  coast-lines.  They  are  natur- 
ally confined  to  the  latter,  and  to  those  lines  of  old 
sea-level  known  generally  as  "  raised  beaches."  Their 
position  at  the  base  of  old  sea-cliffs  renders  them  liable 
to  early  obliteration,  for  they  tend  to  be  obscured  by, 
and  eventually  to  be  concealed  underneath,  a  talus  of 
dtbris.  They  are  not  singular,  however,  in  that  re- 
spect, for  many  of  the  most  interesting  and  important 
of  the  limestone  caverns  of  Western  Europe  have 
been  hidden  in  the  same  way,  their  discovery  having 
been  the  result  either  of  mere  accident  or  of  patient 
scientific  research. 


CHAPTER  XIV 

BASINS 

BASINS    DUE     TO    CRUSTAL    DEFORMATION — CRATER-LAKES — DIS- 
SOLUTION BASINS LAKES  FORMED  BY  RIVERS — AEOLIAN  BASINS 

DRAINAGE    DISTURBED    BY    LANDSLIPS GLACIAL  BASINS   OF 

VARIOUS     KINDS,    AS     IN    CORRIES,     MOUNTAIN-VALLEYS,     LOW- 
LANDS,    AND     PLATEAUX ICE-BARRIER      BASINS — SUBMARINE 

BASINS    OF    GLACIAL    ORIGIN. 

ALL  the  varied  topographical  features  of  the  land 
owe  their  origin  either  to  subterranean  or  to 
superficial  agents,  or  to  both.  This  is  true  of  eleva- 
tions and  depressions  alike.  It  would  seem  possible, 
therefore,  to  classify  hollows  according  to  the  mode 
of  their  formation.  Not  a  few,  however,  are  of  com- 
plex origin,  having  resulted  partly  from  hypogene 
and  partly  from  epigene  action.  Indeed,  we  might 
group  all  basins  roughly  in  two  divisions,  according 
as  they  owe  their  origin  more  or  less  directly  to 
crustal  deformation  and  fracture,  or  to  the  action  of 
surface-agents.  Epigene  action,  however,  is  so  mani- 
fold and  diverse — the  agents  of  erosion,  of  transport, 
and  accumulation  act  in  so  many  different  ways — that 
a  more  detailed  grouping  is  desirable.  Any  classifi- 
cation adopted  must  be  more  or  less  arbitrary  and  in- 

278 


jBASINS  279 

complete,    but    it   will    serve  our  purpose   to  group 
basins  as  follows  :— 

1.  Tectonic  basins. 

2.  Volcanic        " 

3.  Dissolution  " 

4.  Alluvial 

5.  ^olian 

6.  Rock-fall 

7.  Glacial 

i.  Tectonic  Basins.  These  owe  their  origin  di- 
rectly to  deformation  of  the  earth's  crust,  whether 
the  result  of  warping  or  of  fracture,  or  both.  In  this 
class  are  included  many  inland  seas,  and  most  of  the 
larger  lakes  of  the  globe.  The  Aralo-Caspian  de- 
pression, with  its  numerous  sheets  of  water  and  de- 
siccated basins,  the  Dead  Sea,  Issyk-Kul,  the  lakes  of 
Equatorial  Africa,  the  Great  Salt  Lake  of  Utah,  and 
very  many  others  are  true  tectonic  basins.  A  large 
number  of  such  basins  occur  in  relatively  dry  and 
rainless  regions.  On  the  other  hand,  many  are  met 
with  in  temperate  regions.  The  great  fresh-water 
lakes  of  North  America  and  Europe  (Superior, 
Huron,  Michigan,  Ladoga,  Onega,  etc.)  occupy  tec- 
tonic basins.  These  lakes,  it  will  be  noted,  are 
confined  to  the  glaciated  areas  of  the  two  continents, 
and  their  character  as  tectonic  basins  has  been  modi- 
fied and  obscured  by  glacial  erosion  and  accumula- 
tion. There  seems  no  reason  to  doubt,  however,  that 
the  depressions  are  the  result  of  crustal  deformation. 
Tectonic  basins  are  usually  somewhat  flat-bottomed  or 


280  EARTH  SCULPTURE 

gently  undulating.  Occasionally  they  are  traversed 
by  narrow  winding  hollows,  which  have  been  traced 
for  longer  or  shorter  distances.  These  have  fre- 
quently the  character  of  river-ravines  and  valleys,  and 
are  suggestive,  therefore,  of  a  former  land-surface 
which  has  become  depressed.  Similar  indications  of 
depression  are  afforded  by  the  highly  indented  coast- 
lines of  some  of  the  larger  lakes  of  this  class,  the 
long  inlets  and  projecting  headlands  recalling  the  ap- 
pearances presented  by  the  fiord-coasts  of  Norway 
and  Scotland. 

The  crustal  deformation  may  consist  of  simple 
subsidence — a  wide  area  of  relatively  flat  or  gently 
undulating  land  sinking  below  the  level  of  adjacent 
tracts  ;  or  the  subsidence  may  be  the  effect  of  dis- 
location and  displacement.  Again,  basins  have  come 
into  existence  between  contiguous  high  grounds  un- 
dergoing elevation.  Once  more,  the  formation  of 
an  anticline  across  the  drainage-area  of  a  lowland 
region  might  bring  extensive  lakes  into  existence. 
Similarly  it  is  conceivable  that  lakes  might  be  formed 
in  mountain-valleys  by  the  swelling  up  of  the  crust  at 
the  base  of  the  mountains,  or  by  the  formation  of  new 
flexures  in  the  mountains  themselves,  having  a  direc- 
tion transverse  to  the  valleys.  We  cannot,  however, 
point  to  any  particular  valley-basin  formed  in  this  way. 
Earth-movements  of  this  kind  would  seem  to  take 
place  very  slowly,  so  slowly,  as  a  rule,  that  rivers  are 
able  to  saw  across  the  obstructions  as  fast  as  they 
rise. 


BASINS  281 

2.  Volcanic  Basins.  The  lakes  of  this  class  form  a 
well  marked  group,  many  of  them  occupying  the  sites 
of  extinct  volcanoes.  Not  a  few,  therefore,  occur  in 
the'cup-shaped  depressions  of  'volcanic  cones.  Others, 
again,  may  not  be  walled  round  by  volcanic  ejecta, 
but  occupy  explosion-craters — the  more  or  less  deep 
concavities  produced  by  paroxysmal  outbursts.  No 
hard-and-fast  line,  however,  can  be  drawn  between 
these  two  varieties  of  crater-lake.  Some  explosion- 
craters  are  encircled  by  ridges  of  ejecta,  while  the 
cup-shaped  depressions  of  certain  volcanic  cones  are 
of  such  a  depth  that,  were  the  cones  themselves  to  be 
removed,  a  considerable  concavity  would  still  remain. 
Amongst  well  known  crater-lakes  are  the  Maars  of 
the  Eifel,  some  of  which  are  70  feet  or  less  in  depth, 
while  others  are  not  much  below  200  feet.  Of  the 
same  character  are  the  crater-lakes  of  Auvergne, 
which  vary  in  depth  from  less  than  100  to  350  feet; 
and  the  similar  lakes  of  Central  Italy,  one  of  which, 
Lake  Bracciano,  is  said  to  be  950  feet  deep.  In  all 
the  great  volcanic  regions  of  the  globe,  indeed,  lakes 
of  this  character  are  recognised.  Other  volcanic 
lakes  have  had  a  different  origin.  Sometimes  lava, 
at  other  times  fragmental  ejecta,  or  streams  of  tu- 
faceous  mud  and  debris  have  entered  valleys  and 
obstructed  the  drainage.  The  Lac  d'Aydat  of  Au- 
vergne, for  example,  is  confined  by  a  barrier  of  lava, 
and  the  same  is  the  case  with  the  large  Yellowstone 
Lake.  So,  again,  the  enormous  torrents  of  mud  and 
debris  which  poured  down  to  the  low  grounds  during 


282  EARTH  SCULPTURE 

the  great  eruption  of  Bandaisan  in  1888  gave  rise  to 
four  volcanic  barrier-lakes.  After  volcanoes  have 
erupted  for  a  prolonged  time  the  ground  often  be- 
comes depressed,  and  large  and  small  subsidences  of 
the  surface  are  not  infrequently  the  result  of  the 
earthquakes  that  accompany  volcanic  action. 

3.  Dissolution  Basins.  In  regions  of  soluble  rocks, 
as  we  have  seen,  many  inequalities  of  the  surface 
are  brought  about  by  the  chemical  and  mechanical 
action  of  underground  water.  Most  frequently  the 
depressions  produced  by  the  collapse  of  subterranean 
galleries  and  caves  contain  no  water.  Sometimes, 
however,  as  Professor  Penck  has  pointed  out,  warp- 
ing of  the  crust  has  brought  the  corroded  and  tun- 
nelled limestone  rocks  under  the  influence  of  the 
subterranean  water-level,  so  that  sink-holes  and  other 
superficial  depressions  have  become  more  or  less 
deeply  filled.  Again,  should  tectonic  movements 
carry  down  a  honeycombed  calcareous  region  so  that 
its  basal  portions  sink  below  the  sea-level,  the  mete- 
oric water  descending  from  the  surface  will  be  dammed 
back  in  sinks  and  other  hollows.  The  water-surface 
of  wells  in  such  districts  is  known  to  rise  and  fall  with 
the  tide.  From  various  causes,  also,  the  underground 
outlets  of  dolinas,  etc.,  become  closed  with  accumula- 
tions of  insoluble  earthy  materials,  and  the  bottoms 
of  other  depressions  are  rendered  impermeable  by 
similar  deposits  washed  into  them  by  rain-  or  snow- 
water. Similar  changes  have  been  brought  about  by 
glacial  action,  the  outlets  for  the  escape  of  under- 


BASINS  283 

ground  water  having  been  closed  by  morainic  debris. 
For  these  and  other  reasons  lakes  are  by  no  means 
always  wanting  in  regions  of  highly  honeycombed  and 
tunnelled  calcareous  rocks. 

Soluble  rocks  deeply  covered  with  strata  of  more 
durable  character  do  not  escape  corrosion,  but  are 
gradually  removed  by  underground  water,  and  thus 
bring  about  slow  subsidence  or  sudden  collapse  of  the 
surface,  and  the  shallow  basins  formed  in  this  way 
may  become  filled  with  water. 

4.  Alluvial  Basins.  The  broad  alluvial  flats  of 
rivers  often  show  slight  depressions  caused  by  irregu- 
lar accumulation.  These  during  floods  may  become 
lakes,  and  endure  for  a  longer  or  shorter  time.  Again, 
rivers  tend  to  change  their  courses,  and  their  deserted 
loops  often  persist  as  lakes.  In  rainless  regions  the 
rivers  flow  with  a  gradually  lessening  volume,  until 
eventually  they  may  dry  up.  It  is  obvious  that  the 
sediment  transported  by  such  rivers  must  gradually 
raise  the  level  of  their  lower  courses,  and  in  time  pro- 
duce shallow  basins.  In  the  dried-up  courses  them- 
selves pools  and  "  creeks "  not  infrequently  occupy 
the  deeper  hollows,  and  are  probably  maintained  by 
water  coming  from  underground  sources.  Once 
more,  in  well  watered  regions  rivers  now  and  again 
form  lakes.  A  main  stream,  for  example,  by  carrying 
down  large  quantities  of  detritus,  tends  to  raise  the 
surface  of  its  bed  above  that  of  its  tributaries,  in  the 
lower  reaches  of  which  lakes  thus  come  into  exist- 
ence. In  like  manner  tributary  streams  occasionally 


284  EARTH  SCULPTURE 

throw  more  detritus  into  the  main  valley  than  the 
river  in  the  latter  can  at  once  dispose  of.  Partial 
dams  are  thus  produced,  and  large  valley-lakes  form 
above  the  obstructions,  of  which  the  Silser  See  and 
Silvaplana  See  in  Upper  Engadine  are  examples. 

5.  sEolian  Basins.     Another  class  of  basins  owe 
their  origin  to  the  action  of   the  wind.     Some  are 
erosion-basins  caused  by  the  removal  of  loose,  weath- 
ered rock-material.     Professor  Pumpelly  seems  to  have 
been  the  first  to  recognise  basins  of  this  kind,  which 
were  observed  by  him  in  Mongolia.     They  have  since 
been  encountered  in  many  other  regions,  as  in  Bahia, 
in  Central  Asia,  and  elsewhere.       Sometimes  these 
basins  form  temporary  lakes,  at  other  times  the  water 
remains   more  or   less  persistently.     Some    interest- 
ing  examples   have   been   described  by  Mr.   G.    K. 
Gilbert  as  occurring  in  Arkansas  and  elsewhere  in  the 
Great  Plains  of  North  America.     Basins  of  this  kind 
are  naturally  confined   to  relatively  dry  regions — to 
regions  where  the  rocks  and  soils  are  not  sufficiently 
protected   by   vegetation.     Reference    may   also    be 
made   to    the    temporary   or   more   persistent   lakes 
which  owe  their  origin  to  the  unequal  distribution  of 
wind-blown    accumulations,   some  account   of   which 
has  already  been  given. 

6.  Rock-Fall  Basins.     Rock-falls  and  landslips  not 
infrequently  disturb    local  drainage,  and  may  cause 
lakes  to  appear.     Many  small  lakes  of  this  class  oc- 
cur in  the  Alps  and  other  mountain  regions  where  the 
geological  structures  are  weak  and  liable  to  collapse. 


3ASINS  285 

7.  Glacial  Basins.  The  basins  coming  under  this 
head  are  essentially  of  two  kinds.  Some  are  hollows 
of  excavation,  others  owe  their  origin  to  the  unequal 
heaping  up  of  glacial  and  fluvio-glacial  deposits.  It 
is  not  always  possible,  however,  to  distinguish  sharply 
between  the  two.  In  many  cases  excavation  and 
accumulation  have  alike  been  concerned  in  their 
formation.  The  glacial  origin  of  both  is  at  once 
suggested  by  the  fact  that  they  are  confined  to 
regions  which  yield  other  and  independent  evidence 
of  former  glacial  action.  We  note  further  that  their 
presence  has  no  immediate  or  direct  connection  with 
the  character  of  the  rocks  or  with  the  geological 
structure  of  the  tracts  in  which  they  lie.  They  occur 
in  crystalline,  igneous,  and  schistose  rocks,  and  in 
sedimentary  strata  of  all  kinds  and  of  all  degrees 
of  induration — conglomerate,  sandstone,  greywacke, 
clay-slate,  shale,  limestone,  gravel,  etc.  They  are 
not  restricted  to  areas  of  folded,  contorted,  and 
fractured  rocks,  but  appear  with  all  their  character- 
istic features  equally  well  developed  in  places  where 
the  strata  are  gently  undulating  and  approximately 
horizontal. 

The  formerly  glaciated  areas  of  the  earth's  surface 
are  pre-eminently  the  lake-lands  of  the  world.  We 
have  only  to  look  at  a  series  of  good  maps  to  see 
that  this  is  the  case.  Taking  Europe  as  an  example, 
we  find  that  very  few  lakes  occur  in  regions  over 
which  ice-sheets  and  glaciers  have  not  at  one  time 
extended,  the  most  notable  of  those  lakes  being  the 


286  EARTH  SCULPTURE 

volcanic  basins  of  Auvergne,  the  Eifel,  and  Central 
Italy.  What  non-glaciated  region  of  our  continent 
can  show  a  lake-dappled  surface  like  Finland  ? 
Where  in  extraglacial  tracts  can  we  find  anything  to 
compare  with  the  pay  sage  morainique  of  North  Ger- 
many and  Russia  ?  Precisely  the  same  phenomena 
confront  us  in  North  America.  How  abundantly  are 
lakes  distributed  over  all  the  vast  tract  formerly  oc- 
cupied by  the  great  inland  ice  !  South  of  the  glacial 
boundaries  they  are  practically  unknown. 

We  note  further  that  the  vertical  distribution  of 
the  class  of  lakes  now  under  consideration  is  not  less 
suggestive  of  their  origin.  Cirque-lakes  and  other 
high-level  lakes  are  not  confined  to  any  one  region, 
they  occur  in  mountain-tracts  all  the  world  over, 
wherever  these  have  formerly  nourished  glaciers. 
Low-lying  valley-lakes  like  those  of  the  Alps  have, 
on  the  other  hand,  a  much  more  restricted  distribu- 
tion. They  abound  in  the  mountains  of  temperate 
latitudes,  where  great  valley-glaciers  formerly  existed, 
but  they  are  looked  for  in  vain  in  the  mountains  of 
the  warmer  zones,  the  lower  reaches  of  whose  valleys 
have  never  been  glaciated.  Again,  in  the  northern 
tracts  of  Europe  and  North  America  glacial  basins 
are  not  even  confined  to  mountain-valleys,  but  occur 
more  or  less  abundantly  over  the  lowlands  that  sweep 
out  from  the  mountains.  In  a  word,  there  is  a  close 
connection  between  glaciation  and  the  development 
of  lake-basins. 

Basins  of  glacial  origin  naturally  vary  much  in  char- 


BASINS  287 

acter,  according  to  their  position  and  the  particular 
mode  of  their  formation.  Some,  as  mentioned  above, 
are  rock-basins,  others  are  barrier-basins,  and  many 
are  partly  both.  It  must  be  added  that  not  a  few 
lakes  met  with  in  glaciated  regions  are  not  of  glacial 
origin.  This  is  particularly  the  case  in  mountain-val- 
leys, where  barrier-basins  have  often  been  formed  by 
rock-falls  and  fluviatile  action.  Glacial  basins  may 
be  roughly  grouped  as  follows  : — 

1.  Cirque  or  Corrie  basins. 

2.  Mountain-valley  basins. 

3.  Lowland  and  Plateau  basins. 

4.  Ice-barrier  basins. 

5.  Submarine  basins. 

I.  Cirqtie  or  Corrie  basins  are  confined  to  mount- 
ain regions.  Frequently  they  appear  as  niche-like 
indentations  on  mountain-slopes  at  high  elevations 
above  the  bottoms  of  the  adjacent  valleys.  At  other 
times  they  are  set  farther  back  from  the  brow  of  a 
valley,  forming  cup-shaped  depressions  in  the  flanks 
of  the  higher  crests  and  ridges.  When  such  is  the 
case  the  water  escaping  from  them  may  flow  for  a 
longer  or  shorter  distance  before  it  reaches  the  ter- 
minal shoulder  of  a  mountain  to  plunge  downwards  to 
the  valley  below.  In  detail,  cirques  vary  in  character 
with  the  nature  of  the  rocks  and  their  geological 
structure.  Many  have  a  crater-like  appearance,  some 
of  the  wider  ones  resembling  the  section  of  a  steep- 
sided  amphitheatre,  while  the  narrower  ones  show 


288  EARTH  SCULPTURE 

more  abrupt  slopes.  Although  now  and  again  the 
converging  slopes  may  be  relatively  smooth  and  not 
so  steep,  yet  as  a  rule  they  are  rugged  and  precipit- 
ous, showing  bare,  gaunt  walls  of  rock,  trenched  and 
furrowed  by  torrent  action  and  shattered  by  frost.  In 
regions  which  have  formerly  supported  glaciers  cirques 
are  more  or  less  flat-bottomed,  or  saucer-shaped,  and 
consequently  many  are  occupied  by  lakes.  It  is 
worthy  of  note  that  such  corrie-lakes,  or  tarns,  are 
usually  deeper  in  proportion  to  their  extent  than  the 
large  valley-lakes  of  lower  levels.  Many  corrie-lakes 
rest  in  true  rock-basins  ;  others  seem  to  be  wholly 
dammed  by  moraines  ;  while  yet  others  are  partly 
rock-basins,  partly  barrier-basins.  Not  a  few  have 
been  drained  by  the  water  escaping  from  them  cut- 
ting back  its  channel.  Others,  again,  would  seem  to 
be  filled  up  by  rock-falls  and  the  detritus  and  dtbris 
shot  down  from  the  surrounding  heights.  Many 
cirques,  on  the  other  hand,  have  never  contained 
lakes,  their  flat  bottoms  sloping  gently,  but  continu- 
ously, outwards.  That  cirque-basins  have  been  for- 
merly occupied  by  glaciers  is  shown  by  the  presence  of 
moraines  and  the  frequent  appearance  of  roches  mou- 
tonntes  and  striae,  the  direction  of  which  indicates  an 
outflow  of  ice  from  the  depressions.  These  marks  of 
glacial  action  are  confined  to  the  bottom  of  a  cirque ; 
the  precipitous  rock-walls  show  none. 

The  question  of  the  origin  of  cirque-lakes  has  some- 
times been  obscured  by  confounding  the  origin  of  the 
cirques  with  that  of  the  basins  which  occupy  their 


BASINS  289 

bottoms.  While  some  geologists  have  attributed  both 
to  the  action  of  glacier-ice,  by  others  they  are  believed 
to  be  the  result  of  aqueous  erosion.  The  cirques 
themselves  are  doubtless  in  many  cases  the  work  of 
converging  torrents,  aided  by  frost.  Very  frequently, 
however,  frost,  rather  than  running  water,  has  been 
the  chief  eroding  agent,  as  may  be  seen  in  Norway, 
where,  in  immediate  proximity  to  the  #/#/-line,  cirques 
are  now  being  formed.  The  basin  at  the  bottom  of 
a  cirque,  however,  is  the  work  neither  of  running 
water  nor  of  frost  alone,  but  has  been  ground  out  by 
glacier-ice.  In  the  Highlands  and  the  Southern  Up- 
lands of  Scotland  the  head-waters  of  streams  and  riv- 
ers often  proceed  from  cirque-basins,  especially  in  the 
more  elevated  districts.  Many  of  the  smaller  feeders, 
however,  come  from  cirques  which  have  no  basin,  and 
this  is  particularly  the  case  in  the  less  elevated  portions 
of  the  mountain  regions.  The  origin  of  the  latter  is  ob- 
vious ;  we  see  them  being  formed  at  present.  Springs, 
summer  torrents,  snow-water,  and  frost — all  play  their 
parts.  The  converging  mountain-slopes  direct  the 
drainage  to  one  point,  the  result  being  the  formation  of 
a  more  or  less  abrupt  funnel-shaped  depression  resem- 
bling the  section  of  an  inverted  hollow  cone.  The  form- 
ation of  a  basin  at  the  apex  of  this  inverted  cone  by 
aqueous  action  is  impossible.  The  torrent  escaping 
from  the  cirque  simply  digs  its  channel  deeper,  cuts 
its  way  back,  and  by  its  undermining  action  tends  to 
increase  the  slope  of  the  surrounding  walls.  Add  to 
this  the  action  of  frost  in  splitting  up  the  rocks  and 


290  EARTH  SCULPTURE 

detaching  larger  and  smaller  masses,  and  one  can 
readily  understand  how  a  cirque  must  increase  in  ex- 
tent. Cirques  of  this  character  occur  under  all  con- 
ditions of  climate  and  in  every  mountain  region  of 
suitable  structure,  in  temperate,  subtropical,  and  trop- 
ical zones  alike.1  But  the  flat-bottomed  cirque  is 
restricted  to  regions  which  are  now,  or  have  recently 
been,  subjected  to  glaciation.  Cirque-basins  are 
familiar  features  in  the  Alpine  lands  of  temperate 
latitudes,  and  they  are  met  with  likewise,  but  only 
at  lofty  elevations,  in  the  warmer  zones.  When  a 
mountain  area  was  subjected  to  glaciation,  the  cirques, 
which  occurred  in  immediate  proximity  to  the  snow- 
line,  would  form  admirable  reservoirs  for  the  accumul- 
ation of  snow  and  ntvt,  and  the  formation  of  "  summit 
glaciers."  The  shape  of  a  cirque  would  greatly  favour 
glacial  erosion  by  enabling  the  ice  to  concentrate  its 
grinding  and  disrupting  action  upon  the  point  tow- 
ards which  the  mountain-slopes  converged.  Hence, 
in  time,  the  bottom  of  such  a  cirque  could  not  fail  to 
be  ground  out,  and  the  basin  thus  formed,  owing 
to  the  conditions  that  so  specially  favoured  erosion, 

1  Although  the  true  cirque  usually  presents  the  appearance  of  a  niche-like 
indentation  in  a  mountain-slope,  not  a  few  valleys  terminate  upwards  in  great 
amphitheatre-like  cirques,  the  walls  of  which  are  often  very  steep.  Such 
cirque-valleys  appear  now  and  again  in  our  European  mountains.  As  examples, 
may  be  cited  the  great  cirque  of  Gavarni  in  the  Pyrenees,  the  valleys  of  the 
Hallstadter  See  and  the  Konigs  See,  and  of  the  Trenta  and  the  Wochein 
in  the  Alps,  and  the  great  cirque-valleys  of  Norway,  such  as  that  near  Lunde 
(Jostedalsbrae),  the  precipitous  encircling  walls  of  which  rise  more  than  3000 
feet  above  the  bottom  of  the  valley.  Glen  Eunach  (Cairngorm  Mountains)  is  a 
good  example  of  a  Scottish  valley  with  a  cirque-shaped  head.  Such  great 
cirque -valleys  often  contain  lakes. 


£  A  SINS  291 

would  tend   to  be   relatively  deeper  than  the  rock- 
basins  excavated  in  a  broad  mountain-valley. 

The  vertical  distribution  of  corrie-basins  in  any 
given  tract  of  mountains  shows  that  they  are  closely 
related  to  former  snow-lines.  They  occur  in  belts,  or 
zones,  and  are  not  irregularly  scattered  over  a  whole- 
region.  Amongst  the  Scottish  mountains  two  such 
zones  can  be  recognised.  In  the  lower  part  of  these; 
the  corrie-basins  range  from  1  500  feet  to  2400  feet  or 
thereabouts  ;  in  the  upper  they  occur  between  2400 
feet  and  3400  feet.  Consequently,  the  two  zones  are 
met  with  together  only  among  the  most  elevated 
mountain-groups.  In  the  mountains  of  Middle  Ger- 
many the  zone  of  cirque-basins  lies  between  3000  feet 
and  3500  feet  above  sea-level  ;  and  Professor  Partsch 
has  pointed  out  the  significant  fact  that  the  cirques, 
as  we  follow  them  from  west  to  east,  rise  to  higher 
and  higher  levels,  showing,  as  he  says,  that  the  snow- 
line  of  glacial  times  gradually  ascended  as  it  passed 
eastward  into  the  interior  of  the  continent.  Simi- 
larly in  the  Alps  and  the  Pyrenees,  cirque-basins  oc- 
cur in  definite  zones,  and  form  harmonious  systems 
in  the  several  mountain-groups,  each  zone  marking 
out  a  former  snow-  or 


1  Professor  Penck  gives  the  following  table  to  show  the  relative  heights  at- 
tained by  mountain-lakes  —  the  zones  of  greatest  development  of  high-level 
lakes.  He  includes  in  this  table  not  only  cirque-lakes,  but  many  small  barrier- 
lakes  : 

Norway  .....     1000-1600  metres. 

Hohe  Tatra    .....     1500-2100       " 
Eastern  Alps  (Central  Zone)  .  .  .     1700-2800       " 

Graubunden  Alps       ....     2000-2700       " 


EARTH  SCULPTURE 


It  is  interesting  further  to  note  that  in  North  and 
Middle  Europe  the  cirque-basins  affect  chiefly  the 
mountain-slopes  that  face  the  north  and  north-east. 
Thus  of  78  in  the  uplands  of  Norway,  according  to 
Helland,  50  face  the  north,  while  19  open  towards 
the  east.  So,  again,  Partsch  states  that  of  35  in  the 
mountains  of  Middle  Germany  19  look  north  and 
north-east,  1 3  east  and  south-east,  and  only  3  face  the 
south  and  west.  This  distribution,  as  Penck  remarks, 
is  quite  in  keeping  with  existing  conditions,  for  at 
present  most  snow  accumulates  on  northern  and  east- 
ern exposures.  On  southern  exposures  it  quickly 
melts,  while  from  the  western  declivities  of  the 
mountains  it  is  blown  away  by  the  prevailing  west 
winds. 

2.  Mountain-  Valley  Basins.  This  class  includes  all 
lakes  of  glacial  origin  occurring  in  mountain-valleys 
or  closely  connected  with  these.  In  some  regions 
they  are  seen  only  at  the  very  heads  of  the  valleys, 
which  may  be  cirque-shaped  or  not  ;  elsewhere  they 
appear  towards  the  lower  ends  of  the  valleys,  from 
which  they  now  and  again  extend  into  the  low 
grounds  ;  or  they  may  occur  outside  of  the  mountains 
altogether,  opposite  the  mouths  of  great  mountain- 


Transylvanian  Alps    . 

Pyrenees 

Sierra  Nevada  (Granada) 

Himalaya 

Sierra  Nevada  (S.  Marta) 

Andes  of  Peru 

Andes  of  Chili 

New  Zealand  Alps 


1900-2100  metres 
1800-2400 
2900-3200 
4000-5000 
3900-4000 
4300-4600 
1700-3000 
600-1200 


BASINS 


*93 


valleys.  Many  of  these 
are  rock-basins,  others  are 
barrier-basins,  that  is,  the 
water  has  been  im- 
pounded by  the  unequal 
deposition  of  glacial  and 
fluvio-glacial  detritus. 
The  large  majority,  how- 
ever, partake  of  both  char- 
acters ;  the  lakes  occupy 
rock-basins,  the  lower 
ends  of  which  have  been 
heightened  by  morainic 
and  fluviatile  accumula- 
tions. Many  of  the  lakes 
in  question  attain  a  great 
depth.  Amongst  the 
lakes  of  the  Alps,  for  ex- 
ample, we  find  depths 
of  469  feet  (Zurich),  826 
feet  (Constance),  1013  feet 
(Geneva),  1135  feet  (Gar- 
da),  1341  feet  (Como), 
2800  feet  (Maggiore). 
Similar  relatively  deep 
lakes  occur  in  Scotland. 
Loch  Lomond,  for  in- 
stance, has  an  extreme 
depth  of  630  feet,  and 
Loch  Ness  of  780  feet. 


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294  EAR  TH  SC ULP  T URE 

The  mean  depth  of  such  lakes  often  approaches,  and 
occasionally  even  exceeds,  half  of  the  extreme  depth. 
But  when  we  take  into  account  the  superficial  area 
of  the  lakes,  it  becomes  obvious  that  the  basins  they 
fill  are  mere  shallow  pans  or  troughs.  The  depth  of 
Lake  Como,  for  example,  is  only  i3oth  part  of  its 
length  ;  while  the  Lake  of  Geneva  and  Lake  Garda 
are  respectively  230  and  280  times  longer  than  they 
are  deep.  Again,  the  length  of  Loch  Ness  is  136 
times,  and  that  of  Loch  Lomond  176  times  greater 
than  the  depth. 

The  valley-basins  of  the  Alps  and  other  elevated 
regions  of  Europe  are  of  relatively  recent  age.  Not 
one  is  certainly  known  to  be  of  older  date  than  the 
Glacial  Period.  Further,  they  all  lie  within  tracts 
which  have  been  more  or  less  severely  ice-worn. 
Add  to  this  the  suggestive  fact  that  they  are  distri- 
buted without  any  reference  to  the  geological  struct- 
ure of  the  regions  in  which  they  appear.  The  late 
Sir  A.  C.  Ramsay  was  the  first  to  show  that  such 
basins  had  been  excavated  by  glaciers.  In  the  case 
of  a  glacier,  as  we  have  seen,  erosion  is  carried  on 
throughout  the  whole  extent  of  its  bed.  It  is  obvious, 
however,  that  rock-grinding  and  rock-rupturing  will 
proceed  most  actively  under  the  thickest  mass  of  the 
glacier,  and  the  position  of  this  thickest  part  will  de- 
pend on  the  character  of  the  valley  and  the  number 
and  size  of  the  tributary  glaciers.  After  the  glacier 
has  attained  its  maximum  depth  and  speed  its  thick- 
ness progressively  diminishes,  and  its  rate  of  motion 


£ASfNS  295 

at  the  same  time  gradually  decreases  as  it  flows  on  its 
way.  Under  these  conditions  a  shallow  trough  must 
eventually  be  eroded  in  the  bottom  of  the  valley,  the 
depth  and  extent  of  which  will  have  a  definite  relation 
to  the  importance  of  the  glacier.  Towards  the  ter- 
minal part  of  the  ice-flow  erosion  ceases,  while  accu- 
mulation there  reaches  its  maximum,  morainic  debris 
and  fluvio-glacial  detritus  being  dumped  upon  and 
spread  over  the  valley-bottom,  the  surface  of  which 
may  thus  be  considerably  raised.  Hence,  partly  by 
erosion  under  the  glacier,  and  partly  by  accumulation 
in  the  valley  at  and  below  its  terminal  front,  a  trough 
or  basin  is  formed.  On  the  disappearance  of  the 
glacier,  a  valley-lake  comes  into  existence,  the  river 
escaping  from  which  may  by  and  by  work  its  way 
down  through  the  morainic  and  fluvio-glacial  deposits, 
and  thus  gradually  lower  the  level  of  the  lake,  until  thej 
rock-head  is  reached,  after  which  the  lowering  of  the 
level  becomes  a  much  slower  process. 

Valley-basins  of  the  kind  described  occur,  like 
cirque-basins,  indeterminate  zones.  Just  as  the  latter 
indicate  former  g^z^-lines,  so  the  former  mark  out 
the  limits  reached  by  valley-glaciers.  In  the  loftier 
mountain  tracts  of  temperate  and  northern  regions, 
two  or  more  zones  of  cirque-basins  are  found  rising 
one  above  the  other,  each  zone  representing  a  former 
position  of  the  a/z^-line.  In  like  manner  we  have 
in  the  same  regions  corresponding  zones  of  valley- 
basins,  each  of  which  marks  a  distinct  stage  of  former 
glaciation.  The  basins  in  the  lower  reaches  of  the 


296  EARTH  SCULPTURE 

valleys  and  at  the  base  of  the  mountains  belong  to 
the  period  of  maximum  glaciation,  when  the  snow- 
line  .descended  to  its  lowest  level  ;  while  the  basins 
at  or  near  the  heads  of  the  valleys  are  products  of 
later  epochs,  when  the  snow-line  had  retreated  to 
greater  altitudes. 

The  valley-basins  of  a  great  mountain-range  are 
typically  developed  where  the  valleys  open  freely 
upon  the  low  grounds,  for  under  such  conditions  the 
old  glaciers,  meeting  with  no  obstructions,  could 
readily  creep  outwards  from  their  mountain-fastnesses 
and  deploy  upon  the  Vorldnd. 

Thus,  in  the  case  of  the  Alps,  no  barrier  obstructed 
the  outflow  of  the  glaciers  into  Piedmont  and  Lom- 
bardy,  and  similar  conditions  obtained  along  the 
north  front  of  the  mountains  east  of  the  valley  of  the 
Aar.  It  is  in  those  regions,  therefore,  that  the  lower 
valley-basins  are  best  developed.  The  enormous  sea 
of  ice  that  flowed  down  the  Rhone  Valley,  on  the 
other  hand,  was  dammed  back  by  the  opposing  range 
of  the  Jura,  and  deflected  to  right  and  left.  Hence 
the  basins  excavated  by  that  great  glacier  differ  to 
some  extent  from  the  typical  valley-basins  described 
above.  Round  the  lower  ends  of  the  latter  terminal 
moraines  are  usually  more  or  less  well  developed. 
We  look  in  vain,  however,  for  such  moraines  circling 
round  the  lower  ends  of  the  Lake  of  Geneva  or  Lake 
Neuchatel  and  the  smaller  lakes  in  its  neighbour- 
hood. The  Neuchatel  basin  has  not  been  excavated 
by  an  ordinary  valley-glacier  in  the  usual  way  ;  it  did 


BASINS  297 

not  come  into  existence  under  the  lower  reaches  of 
such  a  glacier.  Its  position  at  the  base  of  the  Jura, 
and  the  direction  of  glaciation  in  its  neighbourhood, 
show  that  it  is  a  true  deflection-basin.  When  a 
glacier  is  obstructed  and  turned  aside  from  the  path 
it  would  follow  did  no  such  obstacle  intervene,  the 
ice  heaps  up,  and  its  erosive  action,  therefore,  be- 
comes intensified,  so  that  a  basin  is  eventually  hol- 
lowed out  in  front  of  the  opposing  barrier.  The 
basin  occupied  by  the  Lake  of  Geneva  is  of  a  more 
complex  structure.  The  upper  portion  of  the  lake, 
which  formerly  extended  up  the  valley  of  the  Rhone 
as  far  as  Bex,  is  comparable  to  one  of  the  lakes  of 
Lombardy  ;  it  is  a  mountain-valley  basin.  The  north- 
ern half,  however,  is  a  deflection-basin,  which,  like 
the  basin  of  Neuchatel,  owes  its  origin  to  the  erosion 
induced  by  the  barrier  of  the  Jura,  which  caused  a 
great  heaping-up  of  ice  between  those  mountains  and 
the  Alps. 

Most  of  the  rock-basins  of  the  Alps  have  been 
more  or  less  modified  by  fluviatile  action.  The  levels 
of  many  lakes  have  in  this  way  been  raised,  and  the 
true  character  of  their  basins  obscured.  Were  all 
the  morainic  and  fluviatile  accumulations  in  their 
neighbourhood  to  be  removed,  the  area  of  some  of 
the  lakes  would  be  considerably  reduced.1  On  the 

1  It  has  been  estimated  that  the  surface  of  Lake  Constance  would  fall  200 
feet,  and  its  area  be  reduced  by  a  third,  were  the  deposits  which  partially  dam 
it  up  to  be  removed.  So,  in  like  manner,  could  we  conjure  away  the  superficial 
accumulations  in  the  plains  of  Lombardy  below  Como,  that  lake  would  lose 
nearly  500  feet  of  its  depth,  and  about  half  of  its  area. 


298  EARTH  SCULPTURE 

other  hand,  not  a  few  were  formerly  more  extensive 
than  they  are  now.  Streams  and  rivers  are  gradually 
pushing  their  deltas  forward  into  the  upper  reaches 
of  a  lake  ;  and  the  same  process  takes  place  in  other 
parts  of  the  same  basin  opposite  the  mouths  of  lateral 
streams  and  torrents,  so  that  in  not  a  few  cases  lakes 
have  been  divided  into  two  or  more.  Again,  very 
many  lakes  have  been  entirely  silted  up. 

We  have  spoken  of  the  rock-basins  which  are  so 
commonly  encountered  towards  the  lower  and  upper 
ends  of  mountain-valleys.  It  must  not  be  supposed 
that  glacially  eroded  basins  occur  nowhere  else  in 
mountain-valleys.  Those  referred  to  may,  indeed,  be 
taken  as  the  normal  types  of  valley-basins  ;  each  has 
been  excavated  under  the  lower  reaches  of  a  glacier, 
the  lateral  and  terminal  moraines  and  fluvio-glacial 
gravels  of  which  usually  appear  in  their  immediate 
neighbourhood.  Rock-basins,  however,  have  been 
eroded  elsewhere  in  the  bed  of  a  glacier,  as  in  the 
case  of  the  deflection-basins  already  described.  These, 
as  we  have  seen,  owe  their  origin  to  the  increased 
erosion  caused  by  notable  obstructions  in  the  path  of 
an  ice-flow.  It  not  infrequently  happens  that  a 
mountain-valley  becomes  constricted  owing  to  the 
mutual  approach  of  its  flanks  ;  the  valley-bottom  ex- 
pands and  contracts  as  the  opposing  mountain-slopes 
recede  or  advance.  When  a  valley  of  this  character 
is  occupied  by  a  glacier  it  is  obvious  that  each  con- 
striction must  form  an  obstacle  in  its  path,  with  the 
result  that  under  the  heaped-up  ice  erosion  will  be 


BASINS  299 

intensified  on  the  bed  of  the  valley  above  the  con- 
striction, and  a  shallow  basin  will  be  ground  out.  On 
the  disappearance  of  the  glacier  a  lake  will  necessarily 
appear,  and  many  such  lakes  occur  in  highly  glaciated 
mountain  tracts ;  frequently,  however,  lakes  of  this 
kind  become  silted  up,  and  their  former  presence  is 
then  only  indicated  by  flat  sheets  of  alluvium.  Again, 
it  is  well  known  that  valley-basins  of  the  normal  type 
often  show  irregular  depths,  and  it  is  not  always  easy 
to  say  how  these  have  originated.  Sometimes  they 
are  the  result  of  valley  constriction,  sometimes  of 
sudden  changes  in  the  direction  of  the  valley,  which 
have  caused  the  ice  to  erode  more  energetically  on 
one  side  than  the  other,  for  the  line  of  most  rapid 
motion  in  a  glacier,  as  in  a  river,  will  shift  from 
the  centre  to  the  side,  or  from  side  to  side,  with 
the  windings  of  its  course.  Again,  inequalities  in  the 
floor  of  a  rock-basin  may  sometimes  be  due  to  the 
unequal  resistance  of  the  rocks.  Nor  must  we  forget 
that  during  its  final  melting  a  glacier  might  dump 
debris  in  a  very  confused  fashion  over  its  bed,  while 
the  subsequent  deposition  of  alluvial  matter  swept 
into  the  lake  at  many  different  points  by  streams  and 
torrents  would  similarly  tend  to  produce  inequalities. 
But  all  valley-lakes,  it  must  be  remembered,  are 
not  rock-basins.  On  the  contrary,  not  a  few  Alpine 
lakes,  and  many  which  occur  in  similar  positions  in 
the  mountains  of  other  lands,  are  true  barrier-basins, 
dammed  up  wholly  by  morainic  or  by  fluvio-glacial 
detritus,  or  by  both.  Again,  numerous  small  lakes 


300  EARTH  SCULPTURE 

and  pools  occur  in  the  cup-shaped  and  irregular  de- 
pressions of  the  paysage  morainique  at  the  base  of  a 
mountain  region.  The  moraines  of  this  region  mark 
the  limits  reached  by  the  larger  valley-glaciers.  One 
of  the  most  typical  localities  for  the  development  of 
small  morainic  lakes  of  the  kind  referred  to  is  the 
dreary  district  of  the  Dombes,  in  the  valley  of  the 
Rhone.  There,  however,  many  of  the  pools  are  of 
artificial  origin,  and  used  as  fishponds  by  the  inhabi- 
tants. But  it  is  the  morainic  character  of  the  ground 
that  makes  this  possible. 

Thus  the  paths  of  the  old  valley-glaciers  are  fre- 
quently marked  by  the  appearance  of  glacial  lakes, 
large  and  small,  and  variously  formed.  Great  valley- 
basins  may  be  restricted  to  the  mountains,  or  may 
extend  for  some  distance  into  the  Vorlander,  or  may 
occur  wholly  outside  of  the  mountains.  Most  of 
these  are  rock-basins,  but  their  depth  has  often 
been  increased  by  accumulations  of  superficial  ma- 
terials. Other  valley-basins  are  essentially  barrier- 
lakes.  Lastly,  beyond  the  lowest  valley-basins, 
generally  well  out  upon  the  low  grounds,  we  encoun- 
ter the  numerous  pools  and  lakelets  of  the  paysage 
morainique. 

3.  Plateau  and  Lowland  Basins.  The  glacial  basins 
we  have  hitherto  been  considering  are  products  of 
the  action  of  individual  glaciers,  small  or  great  as  the 
case  may  have  been.  They  occur,  therefore,  either 
within  mountain-valleys,  or  in  their  proximity.  But 
over  the  wide  tracts  formerly  invaded  by  the  "  inland 


JBASINS  301 

ice"  of  Northern  Europe  glacial  lakes  are  not  con- 
fined to  mountain-valleys  and  the  adjacent  Vor lander, 
but  are  scattered  broadcast  over  plateaux  and  low- 
lands. In  those  regions  two  areas  of  special  lake- 
development  may  be  recognised  :  (i)  An  area  in 
which  glacial  erosion  has  been  in  excess  of  glacial 
accumulation ;  and  (2)  an  area  in  which,  conversely, 
accumulation  has  been  in  excess  of  erosion.  In  the 
former  tracts  roches  moutonntes  abound  ;  the  surface 
is  thus  often  rapidly  undulating.  Low-lying,  round- 
backed  rocks  extend  on  every  side,  while  here  and 
there  the  general  monotony  of  the  landscape  is 
partly  relieved  by  bare  hills  and  now  and  again  by 
bald  mountain-heights,  all  scraped,  bared,  worn,  and 
abraded  by  severe  glacial  action.  In  the  countless 
dimples  and  irregular  hollows  of  the  surface  lakes  of 
all  shapes  and  dimensions  make  their  appearance, 
and  the  presence  of  innumerable  bogs  and  marshes 
show  further  how  many  shallow  sheets*  of  water  have 
been  gradually  obliterated.  The  most  notable  region 
of  the  kind  in  Europe  is  Finland,  a  land  of  lakes. 
But  excellent  examples  occur  in  our  own  islands, 
such  as  the  Outer  Hebrides  and  the  low-lying,  rocky 
coast-lands  of  the  tract  lying  between  Loch  Ewe  and 
Loch  Laxford.  In  North  America  the  particular 
lake-lands  of  which  we  now  speak  are  practically  con- 
fined to  and  nearly  co-extensive  with  the  Dominion 
of  Canada. 

Of   the  basins  developed  in  those  regions   some 
have  been  excavated,  while  others  are  barrier-basins. 


3o2  EARTH  SCULPTURE 

The  distribution  of  the  former  cannot  always  be  satis- 
factorily explained.  We  may  suppose  that  under  a 
general  ice-sheet  some  rocks  would  yield  more  readily 
than  others.  Some  geologists  are  of  opinion  that 
certain  rock-basins  may  be  of  preglacial  origin,  and 
that  all  the  ice  did  was  to  plough  out  the  alluvia  with 
which  such  basins  had  been  filled.  The  hollows 
themselves,  they  think,  may  have  been  caused  by  the 
weathering  and  rotting  of  rock,  and  the  subsequent 
removal  of  the  disintegrated  materials  by  wind  or 
other  superficial  agency.  According  to  others  the 
depressions  may  be  tectonic  basins  filled  up  in  pre- 
glacial times  and  only  re-excavated  by  glacial  action. 
Some  of  the  larger  lakes,  such  as  Lakes  Lodoga, 
Onega,  and  others  in  Northern  Europe,  and  the  Great 
Lakes  of  North  America,  almost  certainly  occupy 
tectonic  basins,  modified  no  doubt  by  considerable 
glacial  erosion  and  accumulation.  But  the  far  more 
numerous  small  rock-basins  of  the  regions  now  under 
review  are  unquestionably  hollows  of  erosion.  Some 
appear  to  have  been  ground  out  in  places  where  the 
rocks  offered  less  resistance  to  erosion,  but  probably 
the  position  of  a  larger  number  has  been  determined 
by  the  form  of  the  ground.  This  is  seen  in  the  fre- 
quent appearance  of  rock-basins  in  places  where  the 
glacial  current  suffered  constriction  or  obstruction. 
Thus  in  broken,  hilly  ground  the  thickness  of  ice 
and  the  rate  of  flow  would  vary  from  place  to  place,  and 
unequal  erosion  of  its  bed  would  follow  as  a  natural 
course.  Not  infrequently  prominent  obstructions 


BASINS  303 

rose  in  its  path,  and  in  front  of  these  deflection- 
basins  were  eroded,  which  usually  extend  in  a  direc- 
tion at  right  angles  to  the  trend  of  the  ice-flow.  If 
the  obstruction  were  an  isolated  hill  or  mountain  the 
hollow  often  assumed  a  horse-shoe  shape,  encircling 
the  base  of  the  hill.  Much  morainic  debris  was  usu- 
ally accumulated  in  the  rear  by  such  an  obstruction, 
so  as  to  form  a  long,  sloping  "  tail."  Again,  valleys 
which  have  chanced  to  coincide  in  direction  with  the 
ice-flow  not  infrequently  show  a  succession  of  two 
or  more  constriction-basins.  In  flat  lands  of  tolera- 
bly even  surface,  however,  deflection-  and  constric- 
tion-basins are  wanting,  the  great  majority  of  the 
lakes  being  drawn  out  in  the  direction  of  ice-flow. 
Although,  owing  to  the  presence  of  glacial  and  other 
superficial  accumulations,  we  cannot  always  be  sure 
whether  such  lakes  rest  wholly  in  rock-basins  or  not, 
there  can  be  no  doubt  that  they  owe  their  origin  to 
glacial  action,  partly  to  erosion  and  partly  to  accu- 
mulation. In  the  low  grounds  of  Lewis  (Outer  He- 
brides) the  multitudinous  lakes  almost  invariably  tend 
to  assume  a  linear  direction,  and  by  far  the  larger 
number  are  arranged  along  one  or  other  of  two  lines, 
which  strike  as  nearly  as  maybe  N.W.  and  S.E.,  and 
N.E.  and  S.W.  respectively.  Not  infrequently  one 
and  the  same  lake  shows  both  lines  of  direction,  one 
portion  of  the  water  trending  at  right  angles  to  the 
other.  Nearly  all  the  longest  and  most  considerable 
lakes  range  from  S.E.  to  N.W.  This  is  the  direction 
of  glaciation,  and  the  lakes  having  this  particular 


^\V>  **  A  £|  -v^l 

OF  THE 

UNIVERSITY 


304  EARTH  SCULPTURE 

trend  rest  sometimes  in  true  rock-basins,  sometimes 
in  hollows  between  parallel  banks  formed  wholly  of 
glacial  deposits,  or  partly  of  these  and  solid  rock. 
The  north-east  and  south-west  lakes,  on  the  other 
hand,  are  drawn  out  more  or  less  at  right  angles  to 
the  path  of  the  old  ice-flow.  They  follow  precisely  the 
line  of  "  strike  "  (or  general  direction  of  the  outcrop- 
ping ledges  or  reefs  of  gneiss)  ;  when  this  direction 
changes  there  is  a  corresponding  change  in  the  trend 
of  the  lakes.  Thus  in  places  where  the  strike  is  east 
and  west  we  have  east  and  west  lakes,  which  wheel 
round  to  south-west  as  soon  as  the  strike  shifts  to 
that  direction.  In  preglacial  times  the  low-lying 
tracts  of  Lewis  were  in  many  places  traversed  by  a 
series  of  rough  ridges  and  interrupted  escarpments, 
with  intervening  hollows  corresponding  to  outcrops 
of  the  harder  and  the  less  resisting  beds  of  gneiss. 
The  dip  of  the  rocks  being  generally  south-east,  the 
escarpments  naturally  faced  the  north-west.  The 
inland  ice,  which  subsequently  overflowed  this  region 
from  south-east  to  north-west,  then  advanced  against 
the  dip-slopes  of  the  gneissose  rocks,  which  were 
ground  bare,  while  bottom-moraine  was  here  and  there 
deposited  in  front  of  the  cliffs,  knolls,  and  rocky  ledges 
and  ridges  formed  by  the  outcrops  of  the  harder 
beds.  Hence,  when  the  ice  finally  disappeared,  the 
hollows  lying  between  parallel  rock-ridges  and  escarp- 
ments were  unequally  coated  with  bottom-moraine, 
and  an  abundant  series  of  longer  and  shorter  troughs 
were  thus  prepared  for  the  reception  of  water.  The 


£  A  SINS  305 

north-east  and  south-west  lakes  are  consequently  bar- 
rier-lakes, dammed  up  wholly  by  boulder-clay  or  with 
rock  and  boulder-clay  together.  The  manner  in  which 
the  two  groups  of  lakes  now  described  frequently 
unite  offers  no  difficulty.  In  many  places  the  old 
strike-ridges  have  been  cut  across  by  the  ice  at  right 
angles,  and  a  new  system  of  ridges  and  hollows  has 
resulted.  And  it  is  not  surprising,  therefore,  to  find 
that  not  only  lakes  but  also  streams  exhibit  both 
directions,  now  trending  north-west  and  south-east, 
and  then  turning  sharply  off  at  right  angles  to  the 
course  previously  followed. 

When  we  leave  the  highly  abraded  and  ice-worn 
regions  of  roches  moutonntes — the  lands  of  multitu- 
dinous lakes  and  lakelets — we  eventually  enter  upon 
tracts  over  which  glacial  accumulation  has  been  in 
excess  of  erosion.  Here  lakes  become  much  less 
numerous,  and  are  met  with  only  at  intervals.  Most 
of  them  extend  over  shallow  depressions  in  the  surface 
of  the  old  ground-moraines,  but  a  few  occupy  rock- 
hollows  ground  out  in  front  of  prominent  obstruc- 
tions. Lakes  of  the  former  kind  were  formerly  much 
more  plentiful,  but  owing  to  their  limited  depths 
many  have  been  silted  up,  and  are  now  replaced  by 
alluvial  flats.  The  deeper  deflection-basins,  on  the 
other  hand,  have  been  more  persistent  as  lakes,  but 
they  are  comparatively  few  in  number.  Passing  still 
farther  outwards,  and  leaving  behind  the  gently  un- 
dulating and  rolling  plains,  throughout  which  ground- 
moraine  forms  the  dominant  deposit  at  the  surface, 


3o6  EARTH  SCULPTURE 

we  reach  at  last  the  paysage  morainique,  with  its  tu- 
multuous hills,  knolls,  ridges,  and  embankments,  and 
find  ourselves  once  more  in  a  region  of  lakes,  or 
rather  of  lakelets,  pools,  and  marshes.  Among  the 
most  conspicuous  examples  of  such  a  region  is  the 
paysage  morainique  of  the  last  great  Baltic  glacier, 
extending  from  west  to  east  through  East  Holstein, 
Mecklenburg-Strelitz,  Uckermark,  Neumark,  South- 
ern Pomerania,  and  the  higher  parts  of  West  and 
East  Prussia.  Another  well  known  region  of  similar 
character  is  the  corresponding  lake-dappled  paysage 
morainique  of  North  America,  which  embraces  such 
vast  tracts  in  the  Northern  States  of  the  Union. 

4.  Ice-Barrier  Basins.  In  existing  glacier-regions 
ice-dammed  lakes  now  and  again  appear.  Of  these 
the  Marjelen  See  on  the  Aletsch  glacier  may  be  taken 
as  an  example.  Their  origin  is  simple  enough. 
When  a  glacier  advances  across  the  mouth  of  a  trib- 
utary valley,  the  stream  flowing  in  the  latter  is 
dammed  back,  and  a  lake  comes  into  existence.  In 
the  Alps  lakes  of  this  kind  have  formed  from  time  to 
time,  the  sudden  bursting  of  the  ice-dams  occasionally 
causing  enormous  devastation.  In  our  own  and  other 
formerly  glaciated  countries  the  relics  of  such  lakes 
—some  of  which  must  have  persisted  for  long  periods 
—are  of  not  infrequent  occurrence.  The  well  known 
"  Parallel-Roads  "  of  Glen  Roy  are  simply  the  beaches 
of  an  ice-barrier  lake. 

5.    Submarine     Basins.      Here    we    are    not    con- 
cerned with  the  large  and  small  basins  that  mark  the 


BASINS  307 

floor  of  the  great  oceanic  troughs,  all  of  which  are 
doubtless  tectonic.  The  hollows  to  which  we  would 
now  refer  are  certain  relatively  smaller  basins  occur- 
ring in  immediate  proximity  to  the  shores  of  recently 
depressed  lands.  The  regions  in  which  they  appear, 
although  submerged,  form,  nevertheless,  a  continua- 
tion of  the  continental  plateau.  The  true  border  of 
the  European  continent,  for  example,  extends  in  the 
north-west  as  far  seaward  as  the  loo-fathom  line  at 
least,  and  there  is  good  ground  for  believing  that 
within  geologically  recent  times  a  large  part,  if  not 
the  whole,  of  that  now  depressed  region  existed  as 
dry  land.  The  sea-lochs  of  Scotland  and  the  fiords 
of  Norway  simply  occupy  old  mountain-valleys,  while 
the  numerous  islets  lying  off  those  coasts  and  the 
British  Islands  themselves  were  all  at  one  time  con- 
nected and  joined  to  the  mainland  of  Europe.  The 
basins  to  which  we  now  call  attention  form  two  more 
or  less  well  marked  groups.  One  of  these  is  practi- 
cally confined  to  the  fiords,  the  other  is  developed 
chiefly  in  front  of  islands  that  face  the  fiords. 

Although  it  is  not  possible  to  go  into  much  detail, 
it  is  nevertheless  necessary  to  indicate  the  character- 
istic features  of  a  typical  fiord  region.  Norway,  as 
we  have  already  learned,  is  an  ancient  plateau,  deeply 
incised  and  cut  up,  as  it  were,  into  irregular  segments. 
These  segments  vary  much  in  extent  and  form — some- 
times the  surface  of  the  fjelcl  is  flat  and  undulating, 
elsewhere  it  is  scarped  and  worn  into  irregular  groups 
and  masses  of  variously  shaped  mountains  and  ridges 


3o8  EARTH  SCULPTURE 

without  any  determinate  arrangement.  The  oro- 
graphy is  everywhere  in  strong  contrast  to  that  of 
the  Alps,  with  their  extended  parallel  chains  and 
longitudinal  valleys.  Not  less  strong  is  the  contrast 
between  the  fiord-valleys  of  Norway  and  the  valleys 
of  the  Alpine  chain.  The  latter  in  cross-section  are 
commonly  V-shaped,  while  the  former  are  U-shaped. 
Again,  fiord  -  valleys  have  relatively  few  lateral 
branches,  the  opposite  being  the  case  with  the  great 
valleys  of  the  Alps,  which  are  joined  by  numerous 
tributaries.  Were  the  Alpine  lands  to  be  so  sub- 
merged as  to  convert  such  valleys  as  the  Rhone  or 
the  Inn  into  arms  of  the  sea,  it  is  obvious  that  numer- 
ous broad  and  long  inlets  would  ramify  right  and 
left  from  these  arms  into  the  mountains.  The  fiord- 
valleys  of  Norway  do  not  branch  after  that  fashion  ; 
the  hydrographic  system  of  the  country,  as  Professor 
Richter  well  observes,  is  imperfectly  developed.  The 
principal  channels  of  erosion  are  the  deep,  trench- 
like  fiord-valleys,  the  tributaries  which  reach  these 
from  the  fjelds  or  plateaux  being  relatively  insignifi- 
cant. The  main  stream,  flowing  through  a  deep 
mountain-valley,  has  cut  its  way  down  to  the  level  of 
the  sea,  which  it  enters  at  the  head  of  a  fiord.  Below 
this  point,  however,  few  or  no  side  valleys,  as  a  rule, 
break  the  continuity  of  the  fiord-walls.  Numerous 
tributary  waters,  some  of  which  are  hardly  less  im- 
portant than  the  head-stream,  do  indeed  pour  into  the 
fiord,  but  they  have  not  yet  eroded  for  themselves 
deep  trenches.  After  winding  through  the  plateau- 


£  A  SINS  309 

land  in  broad  and  shallow  valleys  their  relatively 
gentle  course  is  suddenly  interrupted,  and  they  at 
once  cascade  down  the  precipitous  rock-walls  to  the 
sea.  The  side  valleys  that  open  upon  a  fiord  are 
thus  truncated  by  the  steep  mountain-wall  as  abruptly, 
Dr.  Richter  remarks,  as  if  they  had  been  cut  across 
with  a  knife. 

Mountain-valleys  of  the  V-shaped  Alpine  type  are 
not  wanting  in  the  fjeld,  but  as  they  are  followed  in- 
land towards  the  low  water-partings  of  the  plateau 
they  soon  lose  their  character  and  acquire  softer 
features.  The  valleys  of  the  fjeld-lands  are  for  the 
most  part  broad  and  open,  many  lakes  being  scat- 
tered along  the  courses  of  the  streams.  We  are  here 
dealing,  in  fact,  with  a  plateau  lake-land,  a  region  in 
which  glacial  erosion  has  been  in  excess  of  accumula- 
tion. It  is  through  this  gently  undulating,  highly  ice- 
worn  plateau-land,  with  its  shallow  valleys,  that  the 
profound,  chasm-like  fiord-valleys  have  been  cut  to 
depths  of  3000  to  6500  feet.  That  these  enormous 
gorges  are  the  work  of  erosion  is  not  doubted  by 
geologists,  but  the  problem  of  their  origin  is  never- 
theless complex.  Much  has  been  written  upon  the 
subject,  but  no  one  has  given  a  more  lucid  description 
of  the  actual  facts,  or  a  more  intelligible  explanation 
of  their  meaning,  than  Professor  Richter,  and  him, 
therefore,  we  shall  follow. 

If  we  admit  that  a  fiord  is  simply  a  partially  drowned 
land-valley,  and  that  the  profound  hollow  in  which  it 
lies  has  been  eroded  by  river  action,  how  is  it  that  the 


3io 


EARTH  SCULPTURE 


side  streams  have  succeeded  in  doing  so  little  work? 
Why  should  the  erosion  of  the  main  or  fiord-valleys 
be  so  immeasurably  in  advance  of  that  of  the  lateral 
valleys  ?  Obviously  there  must  have  been  a  time 
when  the  process  of  valley  formation  proceeded  more 
rapidly  along  the  lines  of  the  present  fiords  and  their 
head-valleys  than  in  the  side  valleys  which  open  upon 
these  from  the  fjelds.  At  that  time  the  work  of  rain 
and  running  water  could  not  have  been  carried  on 
equally  over  the  whole  land,  otherwise  we  should  find 
now  a  completely  developed  hydrographic  system— 
not  a  plateau  intersected  by  profound  chasms,  but  an 
undulating  mountain-land  with  its  regular  valleys. 
Nor  can  we  believe  that  the  present  distinctive  feat- 
ures of  fjeld  and  fiord  originated  contemporaneously 
under  a  general  ice-sheet.  The  wild  rock-walls  of  the 
fiords,  mostly  ice-worn  though  they  be,  are  not  glacial 
features.  Ice  does  not  carve  but  canons.  According 
to  Dr.  Richter,  the  remarkable  contrast  between  the 
deep  valleys  of  the  fiords  and  the  shallow  side  valleys 
that  open  upon  them  from  the  fjelds — the  profound 
erosion  in  the  former,  and  the  arrest  of  erosion  on  the 
plateau — admits  of  only  one  explanation.  While 
rivers  and  rapid  ice-streams,  flowing  in  previously  ex- 
cavated valleys,  were  actively  engaged  in  deepening 
these,  the  adjacent  fjelds  were  buried  under  sheets  of 
ntvd.  At  the  time  the  fiords  assumed  their  present 
characteristic  features,  the  snow-line  must  have  been 
depressed  below  its  existing  level,  and  large  glaciers, 
preceded  by  torrential  rivers,  must  eventually  have 


•  flowed  down  the  fiords  to  the  submarine  bars  that 
now  appear  at  or  near  their  entrances.  Such  condi- 
tions obtained  during  certain  stages  of  the  Glacial 
Period,  both  before  and  after  the  epoch  of  maximum 
glaciation.  While  the  fiords  were  being  deepened, 
first  by  rivers  and  thereafter  by  large  glaciers,  the 
fjelds  were  undergoing  effective  glacial  denudation, 
so  that  in  time  their  configuration  became  greatly 
modified.  The  mountain-ridges  with  their  regular 
hydrographic  system,  as  developed  in  preglacial 
times,  were  by  and  by  broken  up  and  replaced  by  the 
undulating  rocky  and  lake-dappled  plateaux  which  we 
now  see.  In  short,  while  rivers  and  glaciers  were 
deepening  the  great  valleys  and  making  their  walls 
steeper,  the  intervening  mountain-heights  were  grad- 
ually being  reduced  and  levelled  by  denudation.  Un- 
derneath the  firn  and  ice  of  the  plateau  the  erosion  of 
deep  gullies  was  at  a  standstill.  It  was  somewhat 
otherwise  in  the  Alps,  where  the  hydrographic  system, 
perfectly  regular  in  preglacial  times,  was  only  slightly 
modified  by  subsequent  glacial  action.  Yet  even  there 
erosion  proceeded  most  rapidly  along  the  chief  lines 
of  ice-flow.  Were  the  great  rock-basins  of  the  prin- 
cipal Alpine  valleys  pumped  dry  we  should  find  the 
mouths  or  openings  of  the  side  valleys  abruptly  trun- 
cated, and  their  waters  cascading  suddenly  into  the 
ice-deepened  main  valleys.  For,  as  Dr.  Wallace  has 

'  shown,  it  is  the  present  \dkz-surface,  not  the  lake- 
bottom,  that  represents  approximately  the  level  of  the 
preglacial  valley.  In  a  word,  erosion  proceeded  most 


3i2  EARTH  SCULPTURE 

actively  in  the  main  valleys,  the  bottoms  of  which 
have  been  lowered  for  several  hundred  feet  below  the 
bottoms  of  the  side  valleys.  Precisely  the  same  phe- 
nomena are  repeated  in  Scotland.  -Were  all  the 
water  to  disappear  from  the  Highland  lakes  and  sea- 
lochs,  we  should  find  waterfalls  and  cascades  at  the 
mouth  of  every  lateral  stream  and  torrent. 

But  another  marked  character  of  the  fiords  has  yet 
to  be  mentioned.  They  are  always  deeper  than  the 
sea  immediately  outside,  usually  very  much  deeper. 
Some  fiords  show  only  one  basin-shaped  depression, 
while  others  may  contain  a  succession  of  troughs. 
Frequently  these  basins  are  confined  to  the  fiord, 
but  in  many  cases  they  extend  for  less  or  greater 
distances  beyond  the  entrance.  In  their  form  and 
disposition  they  are  comparable  to  the  great  valley- 
basins  of  the  Alps  and  similarly  glaciated  mountain 
tracts,  and  there  can  be  little  doubt  that  they  have 
had  a  like  origin.  Were  Scotland  to  be  elevated  so 
far  as  to  run  the  sea  out  of  her  fiords,  the  latter 
would  appear  as  mountain-valleys,  each  with  one  or 
more  considerable  lakes,  in  this  and  other  respects 
exactly  resembling  the  Highland  glens  that  drain 
eastward  into  Loch  Ness  and  the  Moray  Firth.  The 
rock-basins  in  those  glens,  like  the  corresponding 
basins  of  the  Alpine  valleys,  have  often  been  modi- 
fied by  the  accumulation  of  morainic  debris  and  river- 
detritus  at 'their  lower  ends.  Many  Highland  lakes, 
in  short,  are  deeper  than  they  would  be  were  all  the 
superficial  deposits  in  the  glens  to  be  removed.  We 


BASINS  313 

may  well  believe  that  the  same  is  most  likely  to  be 
true  of  the  fiord-basins — the  lips  of  the  basins  may 
in  many  cases  be  buried  to  some  depth  under  mo- 
rainic  debris  and  more  recent  marine  deposits.  But 
that  they  are  true  rock-basins  is  shown  by  the  fact 
that  in  not  a  few  cases  the  sea-floor  at  the  entrance 
is  awash,  ice-worn  rocks  every  here  and  there  rising 
to  the  surface  and  forming  low  islets  and  skerries. 
The  fiord-basins  in  the  depressed  mountain-valleys 
of  Scotland  and  Norway  have  obviously  been  ground 
out  by  large  glaciers  in  the  same  way  as  the  valley- 
basins  of  the  Alpine  lands.  There  are  many  other 
regions  which  show  highly  indented  coasts,  with  long 
inlets  stretching  far  inland,  but  these  do  not  always 
contain  basins.  The  latter  only  appear  in  places 
where  large  glaciers  have  formerly  existed.  Thus 
there  are  no  fiord-basins  in  the  Rias  of  Northern 
Spain,  nor  in  the  inlets  of  the  Istrian  and  Dalmatian 
coasts,  nor  in  the  highly  indented  coast-lands  of  Aus- 
tralia and  South-east  China.  But  basins  are  always 
present  in  the  ice-worn  sounds  of  New  Zealand,  and 
in  the  true  fiords  of  the  higher'  latitudes  of  America. 
In  a  word,  fiords  are  merely  the  drowned  valleys  of 
severely  glaciated  mountain  tracts.  A  very  slight  de- 
pression of  the  land  or  rise  of  the  sea-level  would 
convert  Loch  Maree  and  Loch  Lomond,  and  the 
great  Alpine  valleys  that  open  upon  the  plains  of  the 
Po,  into  typical  fiords. 

Islands,  as  everyone  knows,  are  scattered  more  or 
less  abundantly  along  a  fiord-indented  coast.     Dur- 


3i4  EARTH  SCULPTURE 

ing  the  stage  of  maximum  glaciation  the  glaciers, 
advancing  beyond  the  fiords,  coalesced  in  many  cases 
to  form  a  general  ice-sheet  which  overflowed  those 
islands  in  whole  or  in  part.  It  is  obvious  that  the 
steeper  islands — those  which  rose  more  or  less  ab- 
ruptly above  the  general  level  of  the  sea-floor — must 
have  formed  obstacles  to  the  outflow  of  the  mer  de 
glace.  Some  of  these  mountainous  islets  were  com- 
pletely drowned  in  ice,  while  the  tops  of  others  soared 
above  the  level  of  the  ice-sheet  as  Nunatakkr,  only 
their  less  elevated  portions  being  overwhelmed.  On 
the  sea-floor,  in  front  of  such  islands  we  usually  en- 
counter more  or  less  well  marked  depressions  or 
basins,  some  of  which  attain  a  great  depth.  These 
are  well  indicated  by  the  Admiralty's  charts  of  our 
Scottish  seas.  We  cannot,  of  course,  tell  whether 
those  basins  are  wholly  excavated  in  rock,  or  whether 
they  may  not  owe  some  of  their  depth  to  unequal 
accumulation  of  glacial  and  marine  deposits.  But 
their  form  and  disposition  and  the  whole  configura- 
tion of  the  sea-floor  so  exactly  recall  the  aspect  of 
the  ice-worn  low  grounds  of  the  Outer  Hebrides, 
the  rocky  coast-lands  of  North-west  Scotland,  and  the 
fjelds  of  Norway,  that  we  can  hardly  doubt  that  the 
bottom  of  the  Minch  and  adjacent  areas  owes  its 
characteristic  features  to  glaciation — that  the  deep 
troughs  hugging  the  shores  of  the  rocky  islands  that 
face  the  mainland  are  deflection-basins,  ground  out 
by  the  great  mer  de  glace  on  its  passage  into  the 
Atlantic. 


CHAPTER  XV 

COAST-LINES 

FORM    AND    GENERAL   TREND    OF    COAST-LINES — SMOOTH   OR  REG- 
ULAR    COASTS INFLUENCE    OF     GEOLOGICAL    STRUCTURE    ON 

VARIOUS    FORMS   ASSUMED  BY  CLIFFS CLIFFS  CUT   IN    BEDDED 

AND    IN    AMORPHOUS    ROCKS SEA-CAVES — FLAT    COAST-LINES 

AND    COASTAL    PLAINS — INDENTED    OR    IRREGULAR    COASTS 

GENERAL     TRENDS     OF     COAST-LINES     DETERMINED     BY      FORM 

OF     LAND-SURFACE SUBORDINATE     INFLUENCE     OF     MARINE 

EROSION. 

THE  coast-lines  of  the  globe — the  varied  forms 
they  assume  and  the  directions  they  follow — 
are  an  interesting  study.  Wandering  alongshore 
and  observing  the  effects  of  wave-action,  we  are  soon 
convinced  that  here,  as  in  landward  areas,  hard  rocks 
and  strong  structures  tend  to  resist  erosion,  while 
soft  rocks  and  weak  structures  more  readily  succumb. 
When  we  so  frequently  find  the  former  projecting 
seawards  in  capes  and  headlands,  while  the  latter  are 
often  cut  back  in  bays  and  inlets,  it  might  almost 
seem  as  if  both  the  shape  and  the  direction  of  coast- 
lines had  been  determined  solely  by  marine  action. 
But  this  cannot  be  altogether  true.v'  If  bays  and  all 
other  inlets  and  arms  of  the  sea  were  the  result  of 

315 


316  EARTH  SCULPTURE 

marine  erosion  alone,  the  most  highly  indented  coasts 
should  also  be  the  oldest.  If  not,  then  they  should 
occupy  positions  peculiarly  exposed  to  the  battering 
and  undermining  of  waves  and  breakers,  or  they 
should  be  excavated  in  the  softest  and  most  yielding 
rocks.  The  very  opposite  of  all  this,  however,  is 

i  the  case.  Not  only  are  highly  indented  coast-lines 
of  relatively  recent  age,  but  they  frequently  consist 
of  the  hardest  kinds  of  rock,  and  they  are,  moreover, 
not  subject  to  wave-action  in  any  greater  degree  than 
coasts  which  are  smooth  and  regular.  If  indenta- 
tions were  always  due  to  marine  erosion,  the  sea 
should  be  still  eating  its  way  into  the  land  at  the 
head  of  most  fiords,  estuaries,  and  other  inlets.  In- 
stead of  advancing  in  such  places,  however,  it  is  more 
frequently  receding.  Rivers  entering  the  heads  of 
estuaries  and  sea-lochs  gradually  push  their  deltas 
outwards.  Not  only  so,  but  in  long,  narrow  inlets 
and  fiords  waves  and  breakers  do  very  little  work — 
they  are  practically  powerless.  Since  such  inlets, 
therefore,  are  neither  extended  nor  widened  by  the 
"7  sea,  they  cannot  owe  their  origin  to  its  action.  How- 
ever potent  an  agent  of  erosion  it  may  be,  we  cannot 
credit  it  with  the  formation  of  the  numerous  deep 
indentations  of  such  a  coast  as  that  of  Norway.  In 
point  of  fact,  the  general  tendency  of  marine  erosion 

1  is  to  reduce  irregularities — to  cut  back  headlands,  to 
silt  up  intervening  bays,  and  to  stretch  banks  and 
ridges  across  the  mouths  of  estuaries  and  other  nota- 
ble indentations  of  the  land,  so  as  eventually  to  shut 


COAST-LINES  317 

these  off  more  or  less  completely.  Hence  all  coasts 
which  can  be  shown  to  be  of  relatively  great  age  have 
a  gently  sinuous  or  profusely  curved  outline.  Con- 
versely, as  we  have  indicated,  highly  indented  coasts 
are  of  recent  origin — the  sea  has  not  yet  had  time 
to  reduce  their  irregularities. 

We  must  distinguish  between  the  form  and  the 
general  trend  of  a  coast-line.  The  varying  shape  of 
cliff  and  low  shore  is  no  doubt  largely  determined  by 
the  manner  in  which  the  rocks  yield  to  the  sea,  but 
the  general  direction  followed  by  a  coast  obviously 
depends  on  the  form  of  the  land.  If  the  latter  be 
mountainous,  with  great  valleys  opening  on  the  sea,  i 
the  coast-line  will  usually  be  more  or  less  deeply 
indented.  If,  on  the  other  hand,  it  be  a  low-lying, 
gently  undulating  land,  there  will  be  a  general  ab-V 
sence  of  deep  and  long  inlets,  although  broad  and 
shallow  bays  may  be  numerous.  Such  a  land  may  be 
margined  by  steep  cliffs  or  it  may  be  bordered  by  low 
plains,  or  by  both.  In  short,  however  much  the  sea 
may  modify  the  form  of  its  coasts,  it  is  evident  that  ^ 
it  has  had  but  a  small  share  in  determining  their 
direction.  The  latter  obviously  depends  on  the 
position  of  the  sea-level  and  the  shape  of  the  land. 
Hence  a  very  slight  elevation  or  depression  of  the 
land  would  in  many  cases  completely  change  the 
direction  of  the  coast-lines.  An  elevation  of  30x3  feet, 
for  example,  would  lay  dry  the  bed  of  the  North  Sea 
and  the  English  Channel,  while  an  elevation  of  600 
feet  would  not  only  join  the  British  Islands  to  the 


3 1 8  EARTH  SCULP TURE 

Continent,  but  cause  the  shores  of  Europe  to  advance 
some  50  or  60  miles  beyond  the  Outer  Hebrides  and 
Ireland. 

We  shall  first,  therefore,  treat  of  the  various  forms 
assumed  by  coast-lines,  and  thereafter  the  causes 
which  have  determined  their  general  trends  will  be 
more  particularly  considered.  When  we  run  our  eye 
over  a  map  of  the  world  we  are  struck  by  the  fact 
that  in  some  places  the  coasts  are  relatively  smooth 
and  unbroken,  while  in  other  regions  they  are  more 
or  less  deeply  indented.  We  have  thus  at  least  two 
principal  types,  which  we  may  classify  as  (a)  smooth 
or  regular  coasts,  and  (^)  indented  or  irregular  coasts. 

Smooth  or  Regular  Coasts.  These  may  be  high 
and  steep,  or  low  and  gently  shelving,  the  one  kind 
often  alternating  with  the  other.  (Their  chief  charac- 
teristic is  the  absence  of  prominent  inlets!)  A  steep, 
regular  coast,  as  shown  upon  a  small-scale  map,  has  a 
softly  undulating  or  sinuous  course,  or  presents  a  suc- 
cession of  smaller  and  larger  curves.  It  need  hardly 
be  said  that  when  such  a  long  line  of  cliffs  is  examined 
in  detail,  many  minor  irregularities  make  their  ap- 
pearance. In  some  places  the  cliffs  project  boldly 
beyond  the  average  coast-line  to  form  headlands, 
elsewhere  they  curve  backwards,  or  their  continuity 
may  be  interrupted  by  more  or  less  numerous  creeks, 
gullies,  and  small  inlets,  which  could  only  be  repre- 
sented upon  a  map  of  a  very  large  scale.  ( The  cliffs, 
moreover,  may  vary  in  form  at  relatively  short  inter- 
vals, or  they  may  preserve  great  uniformity  of  char- 


COAST-LINES 


acter  for  long  stretches.  All  such  inequalities  and 
differences  are  due  to  the  nature  of  the  rocks  and  the^ 
mode  of  their  arrangement.  Bedded  rocks,  for  ex- 
ample, owing  to  the  regularity  of  their  joints,  tend  to 
form  cliffs  with  even  faces.  If  the  strata  be  horizon- 
tal, it  is  obvious  that. the  cliffs  must  be  vertical,  or 
nearly  so,  since  the  rocks  naturally  yield  along  their 
approximately  vertical  division-planes.  When  a  slice 
has  been  detached  from  the  cliff,  the  new  surface  ex-1- 
posed  is  an  even  wall  of  rock.  But  as  the  beds 
entering  into  the  formation  of  such  a  cliff  are  likely 
to  yield  unequally  to  weathering,  the  smooth  wall  of 
rock  sooner  or  later  becomes  etched  and  furrowed. 
(Fig.  86.)  Now  and  again,  however,  owing  to  the 


FIG.  86.     SEA-CLIFF  CUT  IN  HORIZONTAL  STRATA. 
jj\  joints. 

nature  of  the  rocks,  or  to  the  rapid  retreat  of  the 
cliffs,  weathering  has  not  sufficient  time  to  effect  any 
marked  modification  of  the  surface.  When  the  strata, 
instead  of  being  horizontal,  are  inclined,  and  the  dip 
is  inland,  or  away  from  the  coast,  the  joint-planes 
necessarily  have  an  inclination  towards  the  sea,  and 
the  cliffs  naturally  slope  in  the  same  direction.  (Fig. 
87,  p.  320.)  On  the  other  hand,  should  the  strata 


320 


EARTH  SCULPTURE 


dip  seaward,  cliffs  hewn  out  of  them  have  a  tendency 
to  overhang,  because  the  division-planes  along  which 


FIG.  87.     SEA-CLIFF  CUT  IN  STRATA  DIPPING  INLAND. 
//,  joints. 

the  rocks  yield  are  now  inclined  away  from  the  shore. 
(Fig.  88.)  Cliffs  having  this  structure  are  in  a  state 
of  unstable  equilibrium — the  truncated  beds  being 
apt  to  slide  forward — so  that  actually  overhanging 


FIG.  88.     SEA-CLIFF  CUT  IN  STRATA  DIPPING  SEAWARD. 
jjt  joints. 

cliffs  of  this  kind  are  not  often  met  with.  Not  infre- 
quently, indeed,  when  strata  dip  seaward  at  a  relatively 
low  angle  they  form  natural  breakwaters,  and  the 
waves  do  not  succeed  in  cutting  out  a  cliff. 

In  all  cases,  when  the  strike  of  the  strata  coincides 


COAST-LINES  321 

approximately  with  the  trend  of  a  coast-line — the  dip 
being  either  seaward  or  landward— the  forms  assumed  / 
by  cliffs  are  largely  determined  by  the  position  of  the 
strike-joints.  The  regularity  of  a  line  of  cliffs  is 
likewise  greatly  controlled  by  the  position  of  the  dip- 
joints,  which,  it  will  be  remembered,  cut  the  strike- 
joints  at  approximately  right  angles.  If  the  former 
be  somewhat  wide  apart,  and  not  strongly  pronounced 
or  discontinuous,  the  sea-wall  may  run  continuously 
for  miles  without  any  marked  interruptions.  On  the 
other  hand,  should  the  dip-joints  be  in  places  more 
numerous  and  closely  set,  they  will  form  lines  of 
weakness,  and  thus  allow  the  waves  to  sap  and  notch 
the  cliff,  so  that  all  such  cliffs  tend  to  assume  rect- 
angular outlines,  the  faces  of  the  sea-wall  and  the  in- 
dentations that  break  its  continuity  being  determined  I 
by  the  double  set  of  joints.  And  the  same  holds 
true  in  the  case  of  horizontal  strata. 

It  goes  without  saying  that  the  cliffs  of  a  regular 
coast  are  evidence  of  marine  erosion.  The  sea  acts 
like  a  great  horizontal  saw,  forming  rock-shelves  and 
terraces  that  increase  in  width  as  the  cliffs  are  under- 
mined and  cut  back.  So  effectually  has  the  work 
been  done  in  many  cases  that  at  high  tide  tfrese 
terraces  of  erosion  are  completely  covered.  Fre- 
quently, however,  islets,  stacks,  and  low  reefs  and 
skerries  appear — fragments  of  land  which  owe  their 
preservation  to  the  superior  hardness  of  the  rocks  at 
the  sea-level,  or  to  some  peculiarity  of  structure,  such 
as  the  paucity  or  absence  of  joints.  Lofty  stacks  are 


322  EARTH  SCULPTURE 

perhaps  most  commonly  met  with  in  the  case  of 
horizontal  or  approximately  horizontal  strata,  or  of 
gently  inclined  beds,  when  the  strike  coincides  with 
the  general  trend  of  a  sea-wall.  But  smaller  stacks, 
reefs,  and  skerries  are  usually  most  abundant  when  the 
coast-line  cuts  across  the  strike,  and  the  truncated 
rocks  differ  much  as  regards  durability.  Such  a 
coast-line  is  usually  very  ragged  or  frayed  out.  The 
cliffs  are  often  approximately  vertical,  but  usually 
show  many  narrow  and  broader  indentations,  while 
long  parallel  ranges  of  reefs,  skerries,  stacks,  and 
islets  diversify  the  surface  of  the  terrace  of  erosion. 

Of  the  various  forms  presented  by  the  projecting 
bastions  and  towers  of  a  line  of  cliffs,  and  by  the 
islets  and  stacks  of  the  sea-shelf,  it  is  not  necessary  to 
say  more  than  that  these  necessarily  vary  with  the 
nature  of  the  rocks  and  the  geological  structure.  In 
the  case  of  horizontal  strata  they  all  have  a  tendency 
to  assume  pyramidal  or  conical  shapes,  and  similar 
forms  are  usually  seen  in  the  cliffs  of  massive  struct- 
ureless accumulations  like  boulder-clay.  Stacks  built 
up  of  inclined  strata  are  usually  less  regular  in  form. 
With  a  low  dip  the  truncated  beds  are  necessarily 
unstable,  and  the  tendency  to  collapse  is  greater  than 
it  is  in  the  case  of  a  horizontal  arrangement.  But 
with  a  high  dip  the  structure  becomes  more  resisting, 
especially  if  the  beds  be  thick  and  massive.  When 
the  strata  are  folded  we  not  infrequently  find  that 
projecting  headlands,  islets,  and  stacks  coincide  with 
synclinal  arrangements.  In  short,  it  may  be  said 


COAST-LINES  323 

generally  that  the  geological  structures  which  best 
withstand  the  action  of  the  eroding  agents  in  mount- 
ainous and  inland  regions  are  just  those  which  offer 
the  most  resistance  to  the  assaults  of  waves  and 
breakers.  Finally,  it  must  be  borne  in  mind  that 
the  action  of  the  sea  in  the  reduction  of  a  steep  coast- 
line is  always  more  or  less  aided  and  modified  by 
other  epigene  agents.  Were  it  not  for  the  action  of 
springs  and  frost  coast-cliffs  would  often  be  steeper 
and  more  abrupt  than  they  generally  are,  the  tendency 
being  for  cliffs  of  all  kinds  of  structure  to  become 
benched  backwards.  Overhanging  and  absolutely 
vertical  rock-walls  are  by  no  means  so  common  as 
one  might  suppose  ;  however  steep  a  cliff  may  be,  it 
usually  has  an  inclination  seawards.  The  accompany- 


FIG.  89.    SEA-CLIFF  CUT  IN  BEDS  DIPPING  SEAWARD. 

a.  a,  cliff-face  determined  by  master-joint ;  cliff  may  yield  along  several  joints  in  succession,  as 

at  b-b. 

ing  diagram,  representing  strata  dipping  seawards, 
shows  how  a  cliff  may  be  overhanging  or  not  accord- 
ing as  the  beds  yield  in  a  wholesale  fashion  along  one 
joint-plane,  or  bed  by  bed  along  different  joint-planes. 
The  cliff-face  a — a  coincides  with  a  master-joint.  It 
is  obvious,  however,  that  yielding  may  take  place  ir- 


324  EARTH  SCULPTURE 

regularly  along  different  joints,  and  we  may  have  the 
overhanging  cliff  benched  back  and  replaced  by  the 
sloping  face  b — b. 

Massive  crystalline  igneous  rocks  yield  forms  of  cliff 
that  offer  strong  contrasts  to  cliffs  excavated  in  bedded 
strata.  Owing  to  inequalities  in  their  composition, 
texture,  and  structure,  and  to  the  frequent  irregularity 
of  their  joints,  they  are  prone  to  assume  particularly 
rugged,  broken,  and  bizarre  forms,  amongst  which  we 
may  look  in  vain  for  any  trace  of  the  rectangular 
outlines  so  commonly  present  in  the  case  of  bedded 
rocks.  The  faces  of  the  cliffs  are  very  rarely  approxi- 
mately even,  but  vary  indefinitely,  the  harder  and 
more  sparingly  jointed  portions  projecting,  it  may  be, 
to  form  buttresses  and  bastions,  while  the  softer  and 
more  shattered  portions  are  eaten  away  and  replaced 
by  coves  and  gullies.  Now  and  again,  however,  when 
the  joints  are  more  regular,  as  in  the  columnar  struct- 
ure of  many  basalts,  etc.,  and  the  approximately  rect- 
angular joints  of  certain  granites,  mural  cliffs  may 
appear.  The  crystalline  schists,  again,  exhibit  every 
variety  of  feature.  But  inasmuch  as  their  bedding  is 
usually  more  or  less  highly  inclined  or  contorted,  and 
their  jointing  is  irregular,  they  do  not  often  show  the 
rectangular  forms  that  are  characteristic  of  cliffs  hewn 
out  of  sedimentary  strata.  Their  coast-lines  are  usu- 
ally as  steep  and  rugged  as  those  of  massive  crystal- 
line rocks,  but  they  present  greater  variety  of  forms, 
the  alternation  of  different  kinds  of  schist  and  the 
highly  inclined,  curved,  or  contorted  bedding,  and  ir- 


COAST-LINES  325 

regular  joints  often  giving  rise  to  most  complex  and 
peculiar  features.  Rugged  stacks  and  skerries  are 
very  commonly  present  when  either  massive  crystal- 
line rocks  or  schists  form  the  coast-line. 

Of  the  formation  of  caves  by  marine  action  we  have 
already  spoken.  Caves  are  not  confined  to  any  one 
kind  of  rock  or  rock-structure,  and  naturally  vary  in 
form  and  extent  with  the  character  and  the  arrange- 
ment of  the  masses  in  which  they  are  excavated. 
When  the  rocks  at  the  base  of  a  sea-cliff  are  of  un- 
equal durability  the  undermining  action  of  the  waves 
and  breakers  must  result  either  in  the  formation  of 
caves  or  in  the  irregular  retreat  of  the  sea-wall.  Much 
will  depend  on  the  character  of  the  rocks  above  the 
reach  of  the  tide.  Should  these  be  massive  and  not 
traversed  by  many  joints,  the  conditions  will  be  fa- 
vourable for  the  formation  of  large  caves.  It  is  obvi- 
ous, however,  that  if  well  marked  joints  be  plentifully 
present  the  rocks  cannot  be  undermined  to  any  extent 
before  collapse  takes  place. 

We  may  now  very  shortly  consider  the  appearances 
presented  by  flat  or  gently  shelving,  regular  coast- 
lines. As  a  rule  these  are  softly  sinuous,  showing  a 
succession  of  broad,  evenly  curved  bays  separated 
usually  by  low  capes  and  headlands.  Shores  of  this 
character  are  often  bordered  by  banks  of  beach- 
gravels  and  sand-dunes,  behind  which  not  infrequently 
appear  salt-water  or  brackish-water  lagoons.  In  the 
absence  of  the  latter  we  may  have  a  coastal  plain 
traversed  by  parallel  series  of  old  beach-gravels  and 


326  EARTH  SCULPTURE 

sand-dunes.  Such  coastal  plains  obviously  owe  their 
origin  to  the  action  of  streams  and  rivers,  and  are 
typically  represented  by  those  great  deltas  which  we 
have  referred  to  in  an  earlier  chapter  as  examples  of 
plains  of  accumulation.  But  the  material  carried  by 
rivers  to  the  sea  does  not  always  accumulate  opposite 
their  mouths.  Tidal  currents  often  prevent  the  rapid 
growth  of  deltas  by  sweeping  much  of  the  material 
away  and  depositing  it  alongshore,  so  as  to  form 
gradually  a  far -extended  coastal  -  plain.  The  low 
plains  that  fringe  the  Atlantic  shores  of  the  Southern 
States  of  North  America  consist  in  this  way  of  the 
sediment  brought  down  by  numerous  streams  and 
rivers,  collected  and  redistributed  by  the  sea.  In- 
deed, of  coastal-plains  generally  it  may  be  said  that 
they  are  either  directly  or  indirectly  of  fluviatile  ori- 
gin. The  delta  of  a  great  river  is  the  direct  product 
of  river-action.  Immense  quantities  of  alluvial  mat- 
ter, however,  are  swept  down  to  sea,  and  accumulate 
upon  the  bottom  at  no  great  distance  from  the  shore. 
Should  a  negative  movement  of  sea-level  take  place, 
a  narrower  or  broader  belt  of  sea-floor  then  becomes 
dry  land,  the  new  coastal  plain  having  been  built  up 
chiefly  of  sediment  washed  down  by  streams  and 
rivers.  Coastal  plains  are  thus  not  infrequently  the  re- 
sult of  crustal  movements.  As  showing  the  depend- 
ence of  coastal  plains  upon  the  activity  of  rivers, 
Professor  Penck  has  pointed  out  that  such  plains  are 
invariably  absent  from  coasts  to  which  no  considerable 
streams  and  rivers  descend. 


COAST-LINES  3^27 

In  fine,  as  regards  regular  coast-lines,  we  see  that 
they  are  not  fixed,  but  oscillating,  retreating  in  some 
places,  advancing  elsewhere.  Cliffs,  stacks,  and  sker- 
ries show  us  where  the  land  is  losing,  and  coastal 
plains  where  it  is  gaining.  Much  sediment  washed 
down  from  the  land  comes  to  rest  in  quiet  bays, 
and  these  in  time  tend  to  be  filled  up.  We  note  also 
how  detritus  derived  from  cliffs  and  rocky  headlands 
is  apt  to  be  swept  by  tidal  currents  into  the  same 
quiet  receptacles.  Thus,  while  cliffs  retreat,  the  flat 
shores  of  adjacent  bays  often  advance,  until  a  definite 
relation  between  the  steep  and  low  coasts  has  been 
established.  When  at  last  the  coast-line  presents,  in 
the  words  of  Reclus,  "  a  series  of  regular  and  rhyth- 
mical curves,"  it  may  become  relatively  stable.  But 
by  the  continuous  descent  of  sediment  from  the  land 
and  its  accumulation  along  low  shores,  and  oy  the 
gradual  retreat  of  cliffs  elsewhere,  complete  stability 
is  impossible. 

Indented  or  Irregular  Coasts.  When  we  consider 
the  surface  of  the  earth's  crust  as  a  whole  we  recog- 
nise two  great  areas,  an  oceanic  depressed  region  and 
a  continental  elevated  region,  or,  shortly,  an  oceanic 
basin  and  a  continental  plateau.  The  larger  land- 
masses  are  all  situated  upon,  but  are  nowhere  co-ex- 
tensive with,  this  plateau,  considerable  portions  of 
which  are  under  the  sea-level.  In  regions  where 
existing  coast-lines  approach  the  margin  of  the  conti- 
nental plateau,  they  are  apt  to  run  for  long  distances 
in  one  determinate  direction,  and,  whether  the  coastal 


328  EARTH  SCULPTURE 

land  be  high  or  not,  to  show  a  gentle  sinuosity. 
Their  course  is  seldom  interrupted  by  bold  headlands 
or  peninsulas,  or  by  long  intruding  inlets,  while  fring- 
ing or  marginal  islands  rarely  occur.  Where,  on  the 
other  hand,  the  coast-line  retires  to  a  great  distance 
from  the  edge  of  the  oceanic  basin,  its  continuity  is 
constantly  interrupted,  and  fringing  islands  usually 
abound.  Thus  the  coast-lines  of  West  Africa  owe 
their  freedom  from  deep  indentations,  their  con- 
tinuous direction,  and  general  absence  of  fringing 
islands,  to  their  approximate  coincidence  with  the 
steep  boundary -slopes  of  the  continental  plateau. 
Conversely,  the  irregularities  characteristic  of  the 
coast-lines  of  North-west  Europe,  and  the  corre- 
sponding latitudes  of  North  America,  are  determined 
by  the  superficial  configuration  of  the  same  pla- 
teau, which  in  those  regions  is  relatively  more  de- 
pressed. In  a  word,  coast-lines  are  profusely  indented 
or  not  according  as  they  recede  from  or  approach  the 
edge  of  the  continental  plateau.  Hence  all  highly 
indented  coast-lines  are  evidence  that  the  land  is  sink- 
ing, or  has  recently  sunk,  the  directions  of  the  coast- 
line depending  on  the  form  or  configuration  of  the 
submerged  land.  If  the  region  be  devoid  of  river- 
valleys,  as  most  desert  areas  are,  the  coast-line  will 
show  no  prominent  indentations.  If,  on  the  other 
hand,  it  be  well  watered  and  mountainous,  its  shores 
will  be  interrupted  by  more  or  less  numerous  narrow 
inlets  running  often  far  into  the  land,  while  peninsulas 
and  fringing  islands  will  probably  abound.  The  fiord- 


COAST-LINES  329 

coasts  of  the  higher  latitudes  of  both  hemispheres  are 
typical  examples  of  the  kind.  Indeed,  we  may  say 
that  irregular  coasts  are  dominant  in  the  higher  lati- 
tudes, while  smooth  coasts  are  more  characteristic  of 
lower  latitudes.  Irregular  coast-lines,  however,  are 
by  no  means  restricted  to  high  latitudes,  but  are  met 
with  in  every  zone.  They  abound  in  the  Mediterra- 
nean :  the  whole  east  coast  of  Asia  is  more  or  less 
deeply  indented  and  margined  by  islands,  large  and 
small ;  Australia,  Madagascar,  Brazil,  the  Isthmus  of 
Panama,  and  many  other  tropical  and  subtropical 
lands,  show  in  places  more  or  less  deeply  indented 
coast-lines.  So  widely  distributed,  in  short,  are  such 
coast-lines  that  the  present  would  appear  to  be  a 
period  rather  of  depression  than  of  elevation.  It  is 
true  that  in  the  fiord-coasts  we  usually  meet  with  evi- 
dence to  show  that  the  land  has  recently  risen,  but 
much  greater  uplift  would  be  required  to  restore 
those  regions  to  their  former  level. 

Indented  or  irregular  coasts  are  thus  not  the  result 
of  marine  erosion.  The  fiords  of  high  latitudes  and 
the  narrow  inlets  of  non-glaciated  lands  are  simply 
submerged  land-valleys  ;  the  intricate  coast-lines  of 
such  regions  have  been  determined  by  preceding 
subaerial  denudation.  The  general  trend  or  direction 
of  the  coasts  everywhere,  therefore,  is  the  result  of 
crustal  movements,  the  actual  form  or  character  of 
the  coast-line,  its  regularity  or  irregularity,  depending 
very  largely  on  its  position  with  reference  to  the  true 
margin  of  the  great  continental  plateau.  In  all 


330  EARTH  SCULPTURE 

regions  where  the  marginal  areas  of  that  plateau  are 
depressed  we  find  a  highly  indented  seaboard  and 
numerous  fringing  islands.  Such  is  the  case,  as  al- 
ready remarked,  in  the  northern  latitudes  of  North 
America  and  Europe,  and  the  phenomena  there  are 
repeated  in  the  corresponding  latitudes  of  South 
America.  Again,  the  manifold  irregularities  of  the 
coasts  of  South-eastern  Asia,  and  the  multitude  of 
islands  between  that  continent  and  Australia  and 
New  Zealand,  are  all  evidence  that  the  surface  of  the 
continental  plateau  in  those  regions  is  extensively 
invaded  by  the  sea.  On  the  other  hand,  where  ex- 
isting coasts  approach  the  margin  of  the  plateau, 
they  are,  upon  the  whole,  more  regular,  showing  few 
or  no  important  indentations  or  fringing  islands. 
The  actual  margin,  however — the  zone  where  conti- 
nental plateau  and  oceanic  basin  meet — is  somewhat 
unstable  and  liable  to  movements  of  elevation  and 
depression.  Where  the  latter  kind  of  movement  has 
recently  occurred,  therefore,  inlets  and  gulfs  make  their 
appearance,  as  at  Rio  Janeiro,  on  the  coast  of  Brazil. 
Movements  in  the  opposite  direction,  however,  by 
laying  bare  the  crustal  shelf  of  marine  erosion  and  sedi- 
mentation, only  produce  a  flat  and  regular  shore-line, 
In  fine,  then,  when  we  consider  the  geographical 
development  of  our  lands  and  their  coast-lines,  we 
must  admit  that  crustal  movements  have  played  a 
most  important  rdle.  But  the  inequalities  of  surface 
resulting  from  such  movements  are  universally  modi- 
fied by  denudation  and  sedimentation.  Table-lands 


COAST-LINES  331 

and  mountains  are  gradually  demolished,  and  the 
basins  and  depressions  in  the  surface  of  the  great 
continental  plateau  become  slowly  filled  with  their 
detritus.  Thus  inland  seas  and  lakes  tend  to  vanish, 
inlets  and  estuaries  are  silted  up,  and  the  land  in 
places  advances  seaward.  To  the  action  of  rain  and 
rivers  that  of  the  sea  is  added,  so  that  by  the  com- 
bined operation  of  all  epigene  agents  the  irregularities 
of  coast-lines  tend  to  become  reduced.  This  is  best 
seen  in  regions  where  the  seas  are  comparatively  shal- 
low— where  the  coast-lines  are  withdrawn  for  some 
considerable  distance  from  the  edge  of  the  great 
oceanic  depression.  In  such  shallow  seas  sedimenta- 
tion and  erosion  proceed  apace.  But  when  the  coast- 
lines are  not  far  removed  from  that  depression,  they 
are  necessarily  washed  by  deeper  waters,  and  become 
modified  chiefly  by  erosion. 

"  Should  they  preserve  that  position  for  a  prolonged  period  of 
time,  they  will  eventually  be  cut  back  by  the  sea.  In  this  way 
a  shelf  or  terrace  will  be  formed,  narrow  in  some  places,  broader 
in  others,  according  to  the  resistance  offered  by  the  varying 
character  of  the  rocks.  But  no  inlets  or  fiords  can  result  from 
such  action.  At  most  the  harder  and  less  readily  demolished 
rocks  will  form  headlands,  while  shallow  bays  will  be  scooped 
out  of  the  more  yielding  masses.  In  short,  between  the  narrower 
and  broader  parts  of  the  eroded  shelf  or  terrace  a  certain  pro- 
portion will  tend  to  be  preserved.  As  the  shelf  is  widened  sedi- 
mentation will  become  more  and  more  effective,  and  in  places 
may  come  to  protect  the  land  from  further  encroachment  by  the 
sea.  All  long-established  coast-lines  thus  acquire  a  character- 
istically sinuous  form." 

"  To  sum  up,  then,"  as  we  have   elsewhere  remarked,  "  the 


332  EARTH  SCULPTURE 

chief  agents  concerned  in  the  development  of  coast-lines  are 
crustal  movements,  sedimentation,  and  marine  erosion.  All  the 
main  trends  are  the  result  of  elevation  and  depression.  Consid- 
erable geographical  changes,  however,  have  been  brought  about 
by  the  silting-up  of  those  shallow  and  sheltered  seas  which  in 
certain  regions  overflow  wide  areas  of  the  continental  plateau. 
Throughout  all  the  ages,  indeed,  epigene  agents  have  striven  to 
reduce  the  superficial  inequalities  of  that  plateau  by  levelling 
heights  and  filling  up  depressions,  and  thus,  as  it  were,  flattening 
out  the  land-surface  and  causing  it  to  extend.  The  erosive  ac- 
tion of  the  sea,  from  our  present  point  of  view,  is  of  compara- 
tively little  importance.  It  merely  adds  a  few  finishing  touches 
to  the  work  performed  by  the  other  agents  of  change." 

But  if  it  be  true  that  all  the  main  trends  of  our  coast- 
lines are  the  result  of  crustal  movements,  it  is  no  less 
true  that  many  of  the  indentations  that  break  the 
continuity  of  an  otherwise  regular  coast-line  are  often 
due  to  the  same  cause.  The  general  trend  of  the 
coast-line  of  South  America,  for  example,  from  Per- 
nambuco  to  the  mouth  of  the  River  Plate,  coincides 
with  the  direction  of  the  continental  plateau,  and  may 
be  said,  therefore,  to  have  been  determined  by  crustal 
movements.  The  shores,  however,  have  been  greatly 
modified  by  sedimentation,  and  to  a  less  extent  by 
erosion,  while  the  numerous  indentations  and  islets  at 
and  near  Rio  Janeiro  are  evidence  of  recent  depres- 
sion. In  a  word,  it  holds  true  for  all  the  coast-lines 
of  the  globe  that  not  only  their  general  direction,  but 
their  more  or  less  numerous  indentations,  are  the 
result  of  crustal  movements.  Estuaries,  fiords,  and 
inlets  generally  are  merely  the  seaward  prolongations 


COAST-LINES  333 

of  valleys  and  other  hollows  of  the  land.  The  indent- 
ations due  to  marine  erosion  are  relatively  so  insig- 
nificant, that  they  can  be  rarely  expressed  upon  a  map 
of  small  scale.  It  is  the  form  of  the  land  that  deter- 
mines the  character  of  a  coast-line.  An  indented 
coast-line  is  the  result  of  depression  ;  a  smooth,  flat 
shore  with  no  indentations  is  more  usually,  although 
not  always,  due  to  elevation  or  sedimentation.  But  a 
featureless  desert-land,  smoothed  out  by  aeolian  ero- 
sion and  accumulation,  would  necessarily  be  bounded 
by  an  even  coast-line,  whether  that  coast-line  were 
the  result  of  upheaval  or  depression.  Finally,  the 
coast-lines  of  regions  which  have  remained  for  a  long 
time  undisturbed  by  crustal  movements  tend,  as  we 
have  seen,  to  assume  a  special  form.  Erosion  and 
sedimentation  in  this  case  combine  to  produce  "a 
series  of  regular  and  rhythmical  curves." 

We  have  made  no  reference  to  the  interesting  fact 
that  plants  and  animals  play  a  certain  part  in  the 
formation  of  coast-lines  in  some  regions.  This  is 
only  conspicuous,  however,  in  tropical  and  subtropi- 
cal latitudes.  The  mangrove-tree,  for  example,  which 
flourishes  along  the  margins  of  low,  shelving  shores, 
forms  dense  belts  of  jungle,  which  continue  to  extend 
seaward  until  the  depth  becomes  too  great.  Some 
of  these  jungles-  attain  a  width  of  ten  or  even  of 
twenty  miles,  and  are  in  places  rapidly  extending. 
Professor  Shaler  is  inclined  to  think  that  on  the  coast 
of  Florida  the  trees  may  advance  over  the  sea-floor 
at  the  rate  of  twenty  to  thirty  feet  in  a  century. 


334  EARTH  SCULPTURE 

The  closely  set  roots  and  rootlets  bring  about  the 
deposition  of  sediment,  and  flotsam  and  jetsam  of  all 
kinds  become  entangled,  so  that  eventually  a  low 
mole  is  formed  along  the  swampy  shore,  which  bars 
the  escape  of  rain-water  towards  the  sea,  and  thus 
marshes  capable  of  supporting  fresh-water  plants  and 
various  bushes  and  trees  come  into  existence. 

In  other  warm  seas  coral  plays  a  not  unimportant 
part  in  the  formation  of  new  lands.  Fringing-reefs, 
barrier-reefs,  and  atolls  are  of  great  interest  from 
many  points  of  view,  but  into  the  history  of  their 
formation  we  need  not  enter.  It  is  enough  to  recog- 
nise the  fact  that  shore-lines  now  and  again  owe  their 
very  existence  to  organic  action. 


CHAPTER  XVI 
CLASSIFICATION  OF  LAND-FORMS 

PLAINS     OF      ACCUMULATION     AND     OF     EROSION — PLATEAUX    OF 

ACCUMULATION     AND    OF    EROSION HILLS    AND    MOUNTAINS  ; 

ORIGINAL  OR  TECTONIC,  AND  SUBSEQUENT  OR  RELICT  MOUNT- 
AINS  VALLEYS  ;    ORIGINAL    OR    TECTONIC,    AND    SUBSEQUENT 

OR   EROSION  VALLEYS — BASINS — COAST-LINES. 

WE  have  now  passed  in  rapid  review  the  more 
salient  and  notable  features  of  the  land-sur- 
face, and  have  discussed  the  several  causes  of  their 
origin.  The  present  chapter  may  therefore  be  de- 
voted to  the  classification  of  those  features,  and  will 
serve  as  a  general  summary  of  the  results  arrived  at. 

The  leading  features  to  be  recognised  are  plains, 
plateaux,  hills  and  mountains,  valleys,  basins,  and 
other  hollows  and  depressions  of  the  surface,  and, 
lastly,  coast-lines. 

i.  Plains.  These  are  areas  of  approximately  flat 
or  gently  undulating  land.  It  is  needless  to  say, 
however,  that  plains  almost  invariably  have  a  general 
slope  in  one  or  more  directions.  This,  however,  is 
so  gentle,  as  a  rule,  that  it  is  hardly  perceptible. 
They  are  confined  to  lowlands ;  but  now  and  again, 

335 


336  EARTH  SCULPTURE 

in  the  case  of  very  extensive  areas,  the  surface  of  a 
plain  rises  inland  so  imperceptibly  that  it  may  attain 
an  elevation  eventually  of  several  thousand  feet. 
This,  however,  is  exceptional.  Elevated  flat  lands 
are  usually  termed  plateaux.  Two  kinds  of  plains 
are  recognised,  viz.,  plains  of  acciimulation  andp/ams 
of  erosion.  A  plain  of  accumulation  is  built  up  of 
approximately  horizontal  deposits,  so  that  the  external 
surface  is  a  more  or  less  exact  expression  of  the 
internal  geological  structure.  All  such  plains  tend  to 
become  modified  by  epigene  action.  If  the  plain  be 
at  or  near  abase-level  of  erosion,  rain  and  running 
water  have  little  effect  upon  it,  but  under  certain 
conditions  the  surface  may  be  considerably  modified 
by  the  action  of  the  wind.  If  the  plain  be  traversed 
by  a  great  river,  or  margined  by  the  sea  or  by  an 
extensive  lake,  sand-dunes  may  invade  it  more  or 
less  abundantly.  Many  coastal  plains,  indeed,  have 
been  formed  partly  by  aqueous  sedimentation  and 
partly  by  the  activity  of  the  wind  blowing  sand 
before  it  from  the  exposed  beaches.  The  higher 
a  plain  rises  above  its  base-level  the  more  it  is  sub- 
jected to  aqueous  erosion,  and  the  more  irregular 
and  undulating  does  its  surface  become,  the  nature 
of  the  materials  of  which  it  is  composed  having  no 
small  influence  in  determining  the  character  and  ex- 
tent of  the  denudation.  Other  things  being  equal, 
a  plain  consisting  chiefly  of  impervious  argillaceous 
deposits  is  more  readily  washed  down  than  one  built 
up  largely  of  sand,  shingle,  gravel,  and  other  more 


CLASSIFICATION  OF  LAND-FORMS  337 

or  less  porous  materials.  Many  plains  of  accumu- 
lation are  among  the  richest  and  most  fertile  tracts 
in  the  world,  while  others  (and  these  are  usually  the 
most  extensive)  are  relatively  infertile,  not  a  few 
being  more  or  less  destitute  of  vegetable  covering. 
Among  European  plains  of  accumulation  may  be 
mentioned  the  French  Landes,  the  far-extending  flats 
of  the  Low  Countries,  and  the  grassy  Steppes  of  Hun- 
gary and  Russia.  The  arid  wastes  of  the  Aralo- 
Caspian  depression  and  the  broad  Tundras  of  Siberia, 
the  Prairies  of  North  America,  and  the  Llanos  and 
Pampas  of  South  America,  are  all  more  or  less  plains 
of  accumulation — their  approximately  flat  or  gently 
undulating  surface  is  due  directly  either  to  aqueous 
sedimentation  or  to  wind-action,  or  to  both. 

Not  infrequently,  however,  the  superficial  accumu- 
lations of  such  tracts  are  of  no  great  thickness,  but 
spread  over  and  conceal  old  plains  of  erosion.  A 
plain  of  erosion  is  distinguished  by  the  fact  that  its 
surface  does  not  necessarily  coincide  with  the  under- 
ground structure.  It  is  only  when  the  plain  has 
resulted  from  the  levelling  of  a  series  of  horizontal 
strata  that  external  form  and  internal  structure  can 
agree.  In  the  great  majority  of  cases  no  such  coin- 
cidence occurs.  The  plains  in  question  may  consist 
either  of  horizontal  or  slightly  inclined  and  gently 
undulating,  or  highly  folded  and  contorted,  strata,  or 
they  may  be  composed  largely  or  wholly  of  igneous 
or  of  schistose  rocks.  They  are  the  base-levels  to 
which  old  land-surfaces  have  been  reduced  ;  they  re- 


338  EARTH  SCULPTURE 

present  the  final  stage  of  a  cycle  of  erosion.  Occur- 
ring as  they  usually  do  in  lowlands,  they  are  liable  to 
become  covered  with  alluvial  and  other  deposits,  and 
thus  at  the  surface  often  show  as  plains  of  accumula- 
tion. Now  and  again  they  have  been  submerged 
and  more  or  less  deeply  buried  under  marine  sedi- 
ments, and  thus  when  re-elevated  the  new-born  lands 
present  the  appearance  of  plains  of  accumulation. 
Probably  the  great  majority  of  the  latter  are  merely 
superimposed  on  pre-existing  plains  of  erosion.  The 
wide  low-lying  tracts  through  which  the  larger  rivers 
of  the  globe  reach  the  sea  are  often  plains  of  erosion 
more  or  less  covered  or  concealed  under  alluvial 
deposits. 

2.  Plateaux  or  Table-Lands.  No  hard-and-fast  line 
can  be  drawn  between  plains  and  plateaux.  The 
term  plateau,  however,  is  usually  applied  to  any  flat 
land  of  considerable  elevation  which  is  separated 
from  lowlands  by  somewhat  steep  slopes.  When  a 
plateau  is  built  up  of  horizontal  beds  it  is  described 
as  a  plateau  of  accumulation — external  form  and  inter- 
nal structure  coinciding.  When  such  is  not  the  case, 
when  the  arrangement  of  the  rocks  and  the  general 
shape  of  the  surface  do  not  agree,  we  have  what  is 
termed  a  plateau  of  erosion.  In  a  word,  plateaux  are 
simply  elevated  plains.  But,  standing  as  they  do  at 
a  higher  level,  they  are  necessarily  subject  to  more 
active  and  intense  erosion,  and,  according  to  their 
age,  are  correspondingly  more  deeply  incised  and 
abraded.  Plateaux  of  all  kinds  eventually  become 


CLASSIFICA  TION  OF  LAND-FORMS  339 

cut  up  into  segments,  and  these  progressively  diminish 
in  extent  as  erosion  proceeds.  Every  table-land,  in 
short,  tends  to  acquire  an  irregular  mountainous  as- 
pect. As  examples  of  highly  eroded  plateaux  of 
accumulation  may  be  cited  the  Plateau  of  the  Colo- 
rado, the  Uplands  of  Abyssinia,  and  the  Deccan  of 
India.  Plateaux  of  erosion,  as  might  have  been 
expected,  are  far  more  common,  many  excellent  ex- 
amples occurring  in  our  own  continent,  such  as  the 
highly  denuded  plateaux  of  Scandinavia  and  Scotland 
and  the  plateau  of  Central  France. 

3.  Hills  and  Mountains.  Just  as  we  cannot  sepa- 
rate plains  from  plateaux  by  any  hard-and-fast  line,  so 
we  find  it  impossible  to  distinguish  clearly  between 
hills  and  mountains.  In  general  we  may  say  that  the 
term  hill  is  properly  restricted  to  more  or  less  abrupt 
elevations  of  less  than  1000  ft.,  all  the  altitudes  ex- 
ceeding this  being  mountains.  The  terms,  however, 
are  loosely  used,  for  in  very  lofty  mountain  regions 
eminences  considerably  above  1000  ft.  are  spoken  of 
as  hills,  while  in  low-lying  tracts  heights  of  only  a  few 
hundred  feet  not  infrequently  become  dignified  with 
the  name  of  mountains.  It  is  obvious,  in  short,  that 
just  as  plains  merge  into  plateaux,  so  there  must  be  a 
gradual  transition  from  hills  into  mountains.  For 
purposes  of  classification,  therefore,  it  is  not  neces- 
sary to  distinguish  between  the  latter,  and  we  shall 
treat  of  them  both  under  the  common  head  of  mount- 
ains. From  our  present  point  of  view,  then,  a 
mountain  is  simply  a  more  or  less  abrupt  elevation, 


340  EA R  TH  SCULP  TURE 

or  somewhat  sudden  increase  in  the  slope  of  a  land- 
surface,  and  may  be  of  any  height  from  less  than  one 
hundred  feet  upwards.  It  may  also  be  of  any  extent, 
and  either  isolated  or  more  or  less  closely  associated 
with  other  elevations,  forming  regular  or  irregular 
groups  or  definite  ranges.  Notwithstanding  the 
great  differences  of  elevation,  of  form,  and  of  ar- 
rangement of  hills  and  mountains,  it  is  obvious  that 
all  these  fall  naturally  into  two  divisions,  namely,  (a) 
elevations  which  have  been  formed  as  such  either  by 
epigene  or  by  hypogene  action,  and  (K)  elevations 
which  have  been  carved  out  of  pre-existing  rock- 
masses  by  epigene  action  alone.  To  avoid  periphrasis, 
we  shall  speak  of  these  two  kinds  of  elevations  as 
original  or  tectonic  mountains,  and  subsequent  or  relict 
mountains,  respectively. 

(a)  Original  or  Tectonic  Mountains.  Under  this 
head  come  many  of  the  most  insignificant  as  well  as 
the  majority  of  the  greater  elevations  of  the  globe. 
Some  of  these  have  been  piled  or  heaped  up  at  the 
surface — they  have  grown  into  heights  by  gradual 
accumulation,  and  may  therefore  be  termed  accumu- 
lation-mountains. This  group  naturally  includes  all 
volcanic  cones  and  hills,  geyser  mounds,  mud-volca- 
noes, etc.  Many  of  these,  no  doubt,  are  mere  monti- 
cles  and  hillocks,  but  all  alike  owe  their  origin  to  the 
extrusion  of  materials  from  below  and  the  accumula- 
tion of  these  at  the  surface.  Of  much  less  import- 
ance are  the  eminences  formed  by  the  direct  action 
of  epigene  agents,  hardly  any  of  which  ever  reach  the 


CLASSIFICATION  OF  LAND-FORMS  341 

height  and  dimensions  that  are  usually  associated 
with  the  term  mountain.  Nevertheless,  they  form 
not  infrequently  conspicuous  land-features,  and  can- 
not be  ignored  in  our  classification  of  land-forms. 
Among  them  are  included  morainic  and  fluvio-glacial 
hills  and  ridges  of  every  kind,  sand-dunes,  etc. 

But  by  far  the  most  important  tectonic  mountains 
are  those  which  have  resulted  from  the  flexuring  and 
fracturing  of  the  earth's  crust, — deformation-mount- 
ains, as  they  may  be  termed.  All  the  great  mount- 
ain-ranges of  the  globe  come  under  this  group. 
The  majority  of  these  owe  their  origin  essentially  to 
tangential  pushing  and  crushing ;  they  consist  for  the 
most  part  of  flexed  and  contorted  rocks.  Now  and 
again,  however,  we  meet  with  mountain-ranges  the 
rocks  of  which  may  show  no  conspicuous  folds  and 
flexures.  Ranges  of  this  kind  have  been  determined 
by  series  of  great  parallel  fractures  and  dislocations  of 
the  crust ;  the  ranges  are,  in  short,  vast  rectangular 
blocks  of  strata  which  may  not  otherwise  be  much 
disturbed.  The  Alps,  the  Himalayas,  and  the  Cor- 
dilleras of  America  are  typical  examples  of  deforma- 
tion-mountains composed  of  highly  folded  rocks. 
Dislocations  are,  of  course  common  enough  among 
such  chains  and  ranges,  but  their  distinguishing  char- 
acter is  the  folding  and  contortion — hence  they  are 
termed  folded  or  flexured  mountains.  The  faulted 
ranges  of  the  Great  Basin  (North  America)  are  nota- 
ble examples  of  the  other  kind  of  deformation-mount- 
ains. In  these  ranges  the  strata  are  sometimes 


342  EARTH  SCULPTURE 

horizontal,  or  approximately  so,  but  are  more  usually 
inclined.  Folding  and  flexing  may  be  absent,  or  only 
partially  and  locally  developed.  The  characteristic 
features  of  such  ranges  are  the  great  faults  that 
bound  them,  and  hence  they  may  be  spoken  of  as 
dislocation-mountains.  In  the  same  category  would 
come  the  Horstc  of  German  geologists.  These  are 
mountains  bounded  by  dislocations — they  project 
above  the  general  level  because  the  rocks  surround- 
ing them  have  been  dropped  down  by  faulting.  Un- 
der the  head  of  deformation-mountains  we  may  also 
include  those  gibbosities,  or  prominent  swellings  of 
the  surface,  caused  by  the  intrusion  below  of  masses 
of  molten  matter.  They  are  typically  represented 
by  the  Henry  Mountains  of  Utah  and  the  Elk 
Mountains  of  Colorado,  and  may  be  termed  lacco- 
lith-mountains. 

Of  course,  all  deformation-mountains  are  more  or 
less  denuded,  some  of  them  to  such  an  extent  that 
their  original  configuration  can  only  be  guessed  at. 
But  since  they  owe  their  elevation  above  adjacent 
lowlands  to  crustal  movements,  they  are  entitled  to 
be  classed  as  tectonic  mountains. 

(B)  Subsequent  or  Relict  Mountains.  Mountains 
belonging  to  this  great  class  frequently  form  irregular 
groups, — there  is  often  an  absence  of  arrangement  in 
separate  parallel  or  interosculating  ridges  and  ranges 
such  as  characterises  tectonic  mountains.  This  ab- 
sence of  alignment  or  orientation,  however,  is  by  no 
means  general,  and  is  most  characteristic  of  relict 


CLASSIFICATION  OF  LAND-FORMS  343 

mountains  which  have  been  carved  out  of  horizontal 
and  gently  undulating  strata,  the  strike  of  which  is 
constantly  changing.  When  the  strike  runs  persist- 
ently for  long  distances  in  one  direction,  the  mount- 
ains in  such  a  region  now  and  again  form  more  or  less 
parallel  ranges,  having  the  same  trend  as  the  strike. 

The  direction  and  to  a  large  extent  the  shape  or 
form  of  relict  mountains  are  thus  mainly  determined 
by  the  geological  structure.  They  are  the  more  salient 
portions  of  plateaux  which  are  in  process  of  being  re- 
duced to  some  base-level  of  erosion.  Plateaux  of 
accumulation  are  eventually  cut  up  into  segments, 
which,  progressively  diminishing  in  extent  and  height, 
form  irregular  groups  of  tabular  and  pyramidal  hills 
and  mountains.  Hills  and  mountains  hewn  out  of 
plateaux  of  erosion,  on  the  other  hand,  not  infrequently 
simulate  the  arrangements  that  are  most  characteristic 
of  deformation-mountains.  Should  the  strata  consist 
of  a  thick  series  of  relatively  soft  rocks,  with  here  and 
there  interbedded  rocks  of  a  less  yielding  kind,  all 
dipping  at  a  moderate  angle  in  one  direction,  the  out- 
crops of  the  harder  rocks  eventually  come  to  project 
prominently.  We  thus  have  long  lines  or  ranges  of 
escarpments,  separated  from  each  other  by  parallel 
hollows.  When  the  strata  dip  at  a  high  angle,  how- 
ever, the  outcrops  of  the  harder  rocks  often  form 
series  of  narrower  and  broader  ridges,  rather  than 
well-marked  escarpments  and  dip-slopes,  but  the  ridges 
continue  to  be  separated  by  strike-valleys.  Even 
when  the  rocks  of  a  plateau  are  highly  contorted  and 


344  EARTH  SCULPTURE 

schistose,  they  nevertheless  sometimes  tend  to  be 
carved  into  ridges  and  ranges,  marking  the  outcrops 
of  the  less  readily  reduced  masses.  More  frequently, 
however,  owing  to  the  direction  given  to  the  drainage 
by  the  original  slopes  of  the  surface,  or  to  the  uni- 
form character  of  the  rocks,  or,  it  may  be,  to  complex 
geological  structure,  all  trace  of  any  definite  linear 
arrangement  disappears — parallel  ranges  and  interven- 
ing hollows  are  replaced  by  amorphous  groups  of 
heights  and  irregularly  diverging  or  radiating  valleys. 
This  is  frequently  due  to  the  presence  of  great  masses 
of  plutonic  rocks,  such  as  granite.  Igneous  intrusions 
of  one  kind  or  another,  indeed,  often  play  a  not  un- 
important rdle  in  giving  variety  to  the  surface  of  such 
regions.  Lastly,  we  may  note  that  when  the  flexured 
rocks  of  a  plateau  are  arranged  in  symmetrical  folds, 
the  synclines,  by  offering  greater  resistance  to  denuda- 
tion than  the  adjacent  anticlines,  tend  to  be  developed 
into  synclinal  mountains. 

As  examples  of  tabular  and  pyramidal  relict  mount- 
ains we  may  cite  the  Red  Sandstone  Hills  of  Suther- 
land, Ingleborough  in  Yorkshire,  the  picturesque  and 
often  fantastic  hills  of  Saxon  Switzerland,  the  basalt- 
heights  of  the  Faroe  Islands  and  Iceland,  and  the 
buttes  and  mesas  of  the  Colorado  Plateau.  Through- 
out the  Lowlands  of  Scotland  we  meet  with  diversified 
features,  all  the  elevations  being  of  subsequent  form- 
ation, or  the  result  of  denudation.  The  Lowlands 
are,  in  short,  a  plain  of  erosion,  the  surface  of  which 
has  been  greatly  modified  by  epigene  action.  The 


CLASSIFICA  TION  OF  LAND-FORMS  345 

more  prominent  knolls,  hills,  heights,  and  ranges  of 
all  kinds  mark  the  outcrops  of  the  relatively  hard 
rocks,   which   in   most   cases   are   of    igneous   origin. 
Many  of  the  isolated   knolls    and  abrupt  eminences 
are  the  necks   of   ancient  volcanoes,  and  these  are 
usually  scattered  irregularly  without  reference  to  the 
dip  of  the  surrounding  strata.     Most  of  the  bolder 
crags  and  escarpments,  however,  are  formed  by  the 
outcrops  of  sheets  and  beds  of  basalt,  etc.     As  the 
dip  is   continually  changing,    such   escarpments   face 
almost  every  point  of  the  compass.     When  the  strike 
is  more  persistent  the  outcrops  of  volcanic  and  intrus- 
ive rocks   often  form  considerable  ranges,  such,  for 
example,  as  the  Ochils,  the  Sidlaws,  the  Pentlands, 
the   Bathgate   Hills,  the  Campsie  Hills,  and  others. 
All  these  heights  might  be  termed  escarpment-hills. 
So   again   the  outcrops  of  the  calcareous   Mesozoic 
strata  of  England  form  still  more  persistent  ranges 
of    escarpment-hills,     traversing    the    country    from 
N.N.E.  to  S.S.W.     The  Moors  and  Wolds  of  York- 
shire,   the    Cotswolds,    the  Chiltern    Hills,   and    the 
Downs  are  examples.     In  all  these  cases  the  dip  of 
the  strata  is  moderate.     In  highly  eroded  regions  of 
steeply  inclined  strata  the  surface-features  are  some- 
times regular,  showing  a  succession  of  parallel  mount- 
ain-ranges  with    intervening   hollows.     -Sometimes, 
however,  they  are  more   or  less   irregular,   the  hills 
and  mountains  being  grouped  together  without  any 
trace   of   linear   arrangement.       The    Highlands    of 
Scotland  to  some  extent  illustrate  the  former  class 


346  EARTH  SCULPTURE 

of  relict  mountains,  the  general  trend  of  the  ranges 
and  intervening  depressions  of  certain  areas  being 
S.W.  and  N.E.  In  the  Southern  Uplands  the  same 
linear  arrangement  is  occasionally  apparent,  but 
hardly  so  marked  as  in  some  parts  of  the  Highlands. 
The  difference  is  probably  in  chief  measure  due  to 
the  fact  that  throughout  the  Southern  Uplands  the 
rocks  show  little  variety,  while  in  the  Highlands  the 
reverse  is  the  case,  zones  and  belts  of  very  different 
kinds  of  rock  alternating. 

The  forms  assumed  by  the  relict  mountains  of  a 
highly  denuded  plateau  of  erosion  do  not  necessarily 
differ  from  those  of  similarly  constructed  tectonic 
mountains.  The  folded  mountains  of  a  region  of 
uplift,  after  long-continued  denudation,  eventually  be- 
come greatly  modified,  the  dominant  elevations  no 
longer  coinciding  with  anticlinal  axes,  but  with  the 
outcrops  of  the  more  resisting  rock-masses,  and  now 
and  again  with  synclinal  axes.  Such  highly  modified 
tectonic  mountains,  from  a  certain  point  of  view, 
might  be  described  as  mountains  of  circumdenudation, 
but  it  is  better  to  distinguish  them.  They  should  be 
recognised  as  tectonic  mountains  through  all  the 
various  stages  of  erosion,  until  they  are  reduced  to 
their  base-level.  Should  such  a  plain  of  erosion  be- 
come a  plateau,  the  mountains  eventually  carved  out 
of  it  might  well  repeat  the  forms  and  the  arrange- 
ments of  the  antecedent  tectonic  mountains,  but  they 
would  be  true  relict  mountains — the  dominant  portions 
of  a  highly  degraded  plateau. 


CLASSIFICA  TION  OF  LAND-FORMS  347 

4.  Valleys.  The  term  valley  has  various  significa- 
tions. Usually  we  mean  by  it  the  hollow  through 
which  a  stream  or  river  flows.  But  some  valleys 
contain  no  streams  ;  they  are  mere  elongated  depres- 
sions— either  narrow  or  broad,  shallow  or  deep. 
Naturally,  however,  all  depressions  in  the  surface  of 
a  land  which  is  not  rainless  tend  to  be  filled  or 
traversed  by  running  water.  By  far  the  great  ma- 
jority of  valleys — using  the  word  in  its  widest  mean- 
ing— are  either  the  direct  result  of  erosion,  or  have 
been  greatly  modified  by  it.  Nevertheless,  not  a  few 
valleys  owe  their  origin  to  other  causes.  In  short, 
we  can  recognise  at  least  two  kinds  of  valleys,  viz., 
(a)  valleys  which  have  been  formed  either  by  hypo- 
gene  action  or  by  epigene  action  other  than  that  of 
running  water  ;  and  ($)  valleys  which  are  true  hollows 
of  erosion.  These  we  shall  briefly  describe  as  original 
or  tectonic  valleys,  and  subsequent  or  erosion  valleys. 

(a)  Original  or  Tectonic  Valleys.  Of  these  we 
distinguish  two  kinds — valleys  which  owe  their  origin 
to  the  irregular  accumulation  or  heaping  up  of  ma- 
terials at  the  surface,  and  valleys  which  are  the 
result  of  crustal  deformation.  The  former  class,  or 
constructional  valleys  as  they  may  be  termed,  are  of 
comparatively  little  importance.  They  occur  some- 
times in  volcanic  regions  as  depressions  in  the  surface 
of  the  various  volcanic  accumulations,  or  as  hollows 
separating  adjacent  cones,  sheets  of  lava,  or  heaps  of 
ejecta.  Similarly  the  depression  lying  between  lines 
and  ranges  of  dunes  and  moraines  may  be  termed 


348  EARTH  SCULPTURE 

constructional  valleys.  Sometimes  such  valleys  trend 
for  miles  in  one  and  the  same  direction  ;  more  usually, 
perhaps,  they  are  winding,  short,  and  interrupted.  In 
a  word,  any  hollows  at  the  surface  produced  by  the 
irregular  distribution  of  materials,  whether  by  volcanic 
action  or  by  epigene  action  of  any  kind,  we  should 
class  as  constructional  valleys. 

Of  much  more  importance  are  deformation-valleys. 
Theoretically  we  may  group  these  as  (i)  dislocation- 
valleys  and  (2)  synclinal  valleys.  But  not  infrequently 
a  deformation-valley  has  been  determined  partly  by 
fracture  and  partly  by  flexure,  such  as  the  valley  of 
the  Jordan.  Dislocation-valleys  may  extend  for  long 
distances  between  parallel  faults,  or  they  may  follow 
the  line  of  one  great  dislocation  alone.  Valleys  of 
this  kind  are  approximately  straight  or  gently  curved, 
and  are  of  not  infrequent  occurrence.  The  valley  of 
Glen  App  in  Ayrshire  and  the  great  hollow  traversed 
by  the  Caledonian  Canal  are  good  examples.  The 
valley  of  the  Rhine  between  the  Vosges  and  the 
Black  Forest  is  another.  Synclinal  valleys,  as  might 
have  been  expected,  are  best  developed  in  mountains 
of  recent  uplift,  where  the  surface-features  not  in- 
frequently coincide  more  or  less  closely  with  the 
underground  rock-structure.  Such  valleys  naturally 
trend  in  the  same  general  direction  as  the  mountains 
amongst  which  they  occur. 

Original  or  tectonic  valleys  of  all  kinds  are,  of 
course,  liable  to  modification  by  erosion.  Many  con- 
structional valleys,  it  is  true,  are  dry,  and  in  the 


CLASSIFICATION  OF  LAND-FORMS  349 

absence  of  running  water  may  remain  for  long  periods 
comparatively  unchanged.  But  wherever  rain  falls 
and  water  flows  we  look  for  evidence  of  erosion. 
Hence,  even  the  most  recently  formed  dislocation 
and  synclinal  valleys  show  traces  of  modification.  As 
regards  the  older  dislocation-valleys,  so  great  has 
been  the  amount  of  subsequent  erosion  that  the  val- 
leys as  we  now  see  them  have  obviously  been  ex- 
cavated by  epigene  action.  They  are  simply  hollows 
which  have  been  worked  out  along  lines  of  weakness. 
As  such  dislocations  go  down  to  great  but  unknown 
depths,  they  necessarily  affect  a  vast  thickness  of 
rock.  However  much,  therefore,  these  rocks  may  be 
denuded,  the  fracture  remains  as  a  line  of  weakness, 
and  determines  the  direction  of  erosion.  The  surface 
may  have  been  planed  down  again  and  again  to  a 
base-level,  but  with  each  re-elevation  a  valley  tends  to 
reappear  in  the  same  place.  Synclinal  valleys,  on 
the  other  hand,  are  far  less  persistent.  When  we 
find  a  river  flowing  continuously  along  the  bottom  of 
a  synclinal  hollow,  we  may  usually  feel  assured  that 
the  hollow  is  of  relatively  recent  geological  age. 
To  this,  however,  there  are  occasional  exceptions. 

(fr)  Subsequent  or  Erosion  Valleys.  If  it  be  some- 
times hard  or  even  impossible  to  draw  a  clear  line  be- 
tween original  and  relict  mountains,  it  is  just  as 
difficult  to  separate  tectonic  from  subsequent  val- 
leys. No  doubt  it  is  easy  enough  to  distinguish  be- 
tween a  young  anticlinal  mountain  and  any  relict 
mountain  carved  out  of  a  plateau.  But  even  the 


350  EARTH  SCULPTURE 

youngest  deformation-mountains  have  sometimes 
been  so  denuded  that  they  might  be  classed  as  relict 
mountains.  It  is  the  same  with  valleys.  Dislocation- 
valleys  no  doubt  tend  to  endure  ;  they  occupy  more 
or  less  permanent  lines  of  weakness.  Synclinal  val- 
leys, on  the  other  hand,  soon  become  modified.  The 
mountains  on  either  side  are  weakly  built,  and  are 
thus  prone  to  collapse,  while  the  intervening  synclinal 
structure  offers  stronger  resistance.  The  rivers  no 
doubt  flow  at  first  along  structural  hollows  or  syn- 
clinal troughs,  but  in  time  the  lines  of  drainage  tend 
to  become  modified  ;  a  river  shifts  its  course  as  the 
anticlinal  mountains  are  reduced,  and  the  syncline 
ere  long  ceases  to  form  a  valley.  It  is  not  surprising, 
therefore,  to  find  that  the  strike-valleys  of  a  recent 
mountain-uplift  often  do  not  coincide  with  synclinal 
troughs,  but  are  true  valleys  of  erosion.  It  is  just  in 
such  regions,  however,  where  tectonic  valleys  are  of 
most  frequent  occurrence.  We  can  have  but  little 
doubt  that  all  the  longitudinal  rivers  of  a  recent 
mountain-chain  flowed  at  first  in  true  structural  or 
tectonic  hollows.  Possibly  also  the  transverse  valleys 
of  such  a  chain  may  sometimes  have  been  determined 
by  minor  folds  and  fractures.  In  the  main,  however, 
they  are  the  result  of  erosion. 

In  ancient  plateaux  of  erosion,  composed  of  highly 
flexed  and  faulted  strata,  we  not  infrequently  en- 
counter surface-features  which  recall  those  of  recent 
mountain-chains.  Such  plateaux  often  assume  a 
mountainous  aspect,  and  the  mountains  sometimes 


CLASSIFICATION  OF  LAND-FORMS  351 

exhibit  a  more  or  less  well-marked  series  of  long  par- 
allel ranges  with  intervening  longitudinal  depressions. 
Transverse  valleys  also  can  be  recognised,  but  these 
present  certain  marked  contrasts  to  the  transverse 
valleys  of  a  recent  mountain-chain.  The  latter  are 
generally  arranged  at  approximately  right  angles  to 
the  longitudinal  valleys,  and  are  consequently,  upon 
the  whole,  "of  less  importance  than  these.1  In  a  de- 
nuded plateau  of  erosion,  however,  the  transverse 
valleys  radiate  in  different  directions  from  the  more 
elevated  portions  of  the  plateau,  cutting  persistently 
across  the  parallel  ranges  and  longitudinal  depressions 
at  all  angles,  and  forming  the  highways  of  the  more 
important  rivers.  It  is  only  occasionally  that  the 
larger  rivers  flow  in  the  direction  of  the  strike.  In 
short,  it  becomes  obvious  that  the  trend  of  the  larger 
rivers  in  an  ancient  plateau  of  erosion  has  been  de- 
termined by  the  original  slopes  of  the  surface,  and 
has  only  an  accidental  connection  with  particular  geo- 
logical structures.  In  the  gradual  development  of 
mountain  and  valley,  however,  the  varying  resistance 
offered  by  the  different  kinds  of  rocks  and  rock- 
arrangements  has  naturally  come  into  play.  Hence 
we  find  mountain-ranges  tend  to  be  developed  along 

1  This,  however,  is  only  true  in  a  general  way,  and  is  most  conspicuously  the 
case  when  a  mountain-chain  is  relatively  broad.  In  the  Alps,  for  example,  most 
of  the  larger  and  longer  valleys  are  longitudinal.  In  mountain-chains  of  in- 
considerable width  the  longitudinal  valleys  are  less  broad  and  more  frequently 
interrupted,  and  the  transverse  valleys  become  relatively  more  important. 
Frequently,  indeed,  the  rivers  flowing  from  the  dominant  crests  of  such  chains 
follow  at  first  a  somewhat  zigzag  course — now  running  in  longitudinal  hollows, 
now  crossing  the  strike — until  eventually  they  become  wholly  transverse. 


352  EARTH  SCULPTURE 

the  outcrops  of  the  harder  or  more  durable  rocks. 
And  thus  it  is  obvious  that  the  intervening  parallel 
depressions  or  longitudinal  valleys  must  as  a  rule  be 
of  later  origin  than  the  main  lines  of  drainage  which 
traverse  the  strike  at  all  angles.  In  short,  when  the 
surface-features  of  such  a  denuded  plateau  are  com- 
pared with  the  aspect  presented  by  the  folded  mount- 
ains of  a  recently  elevated  chain,  we  find  that  the 
contrasts  are  much  more  striking  than  the  resem- 
blances. In  the  former  the  main  lines  of  drainage 
are  independent  of  the  geological  structure,  the  larger 
and  more  prominent  valleys  radiating  in  many  differ- 
ent directions,  and  thus  traversing  the  strike  at  all 
angles.  If  they  sometimes  follow  the  strike  it  is  only 
when  that  has  happened  to  coincide  in  direction  with 
the  slope  of  the  ancient  plateau.  In  general,  how- 
ever, the  strike-valleys  of  such  a  region  have  been 
worked  out  by  lateral  streams.  In  the  case  of  a 
broad  mountain-chain  of  recent  uplift,  on  the  other 
hand,  the  main  lines  of  drainage — the  longer  and 
broader  valleys — follow  the  strike,  while  the  narrower 
and  shorter  transverse  valleys  open  into  these  pri- 
marily at  approximately  right  angles.  In  time,  how- 
ever, many  modifications  necessarily  occur,  and  the 
same  streams  and  rivers  are  found  flowing  now  in 
one  direction,  and  now  in  another,  sometimes  follow- 
ing, and  at  other  times  crossing,  the  strike.  In  a 
word,  the  valleys  that  traverse  an  old  plateau  are 
wholly  the  work  of  erosion,  the  direction  of  the  prin- 
cipal drainage-lines  having  been  determined  by  the 


CLASSIFICATION  OF  LAND-FORMS  353 

surface-slopes  of  the  original  plane  of  denudation,  and 
not  by  the  geological  structure.  The  principal  valleys 
of  a  young  mountain-chain,  on  the  other  hand,  coin- 
cided at  first  with  great  structural  hollows  ;  and  not 
a  few  still  follow  the  lines  of  synclinal  troughs  and 
longitudinal  fractures  and  dislocations. 

If  it  be  true  that  the  valleys  of  a  plateau  are  the 
work  of  erosion,  this  is  not  less  true  of  the  river-val- 
leys of  lowland  regions.  The  main  direction  of  the 
drainage  in  such  regions  has  doubtless  been  deter- 
mined by  the  average  slope  of  the  original  surface, 
and  has  no  necessary  connection  with  the  geological 
structure  of  the  underlying  rocks.  But  however  in- 
dependent of  the  general  rock-arrangement  the  aver- 
age direction  of  the  rivers  may  be,  it  is  obvious  that 
their  courses  have  often  been  profoundly  modified  by 
the  nature  and  structure  of  the  materials  through 
which  these  courses  have  been  cut.  Not  only  are 
they  liable  to  frequent  deflection,  but  the  form  and 
character  of  the  valley  constantly  change  as  different 
kinds  of  rock  and  rock-structures  are  traversed.  A 
river  cutting  through  horizontal  strata  or  igneous 
rocks  with  well-marked  vertical  jointing,  is  usually 
flanked  by  approximately  vertical  cliffs.  But  as  the 
valley  is  widened  the  cliffs  tend  to  become  benched 
backwards  or  even  to  be  replaced  by  slopes.  So, 
again,  courses  cut  in  the  direction  of  the  dip,  or 
against  the  dip,  may  be  bounded  on  either  side  by 
steep  cliffs,  which,  under  the  influence  of  epigene 
action,  often  become  resolved  into  slopes  as  the  val- 


354  EARTH  SCULPTURE 

ley  is  widened.  In  all  those  cases  the  valley-cliffs 
and  slopes  on  the  one  side  have  the  same  general  as- 
pect as  those  on  the  other.  But  when  a  river  cuts  its 
way  along  the  strike  of  moderately  inclined  strata  its 
course  assumes  a  different  form.  On  the  one  side 
cliffs,  and  on  the  other,  where  the  rocks  dip  into  the 
valley,  slopes  tend  to  be  developed.  Again,  as  a 
river  in  its  journey  across  a  wide  tract  will  necessarily 
traverse  rocks  and  rock-structures  of  very  different 
degrees  of  durability,  its  valley  will  widen  or  contract 
according  as  the  rocks  are  more  or  less  readily  eroded. 
In  one  place  the  river  meanders  through  a  plain  bor- 
dered by  gentle  slopes,  in  another  place  it  hurries 
through  a  narrow  and  sometimes  approximately 
straight  or  gently  winding  gorge,  the  latter  often  in- 
dicating the  site  of  former  cascades,  waterfalls,  and 
rapids.  In  a  word,  every  change  in  the  form  and 
character  of  a  valley  of  erosion  is  determined  by  the 
nature  of  the  rocks  and  rock-arrangements  with  which 
the  river  and  its  assistant  agents  have  to  deal. 

Waterfalls  frequently  mark  the  outcrops  of  relat- 
ively hard  rock-masses.  The  Falls  of  Niagara,  for 
example,  owe  their  origin  to  the  intercalation  of  a 
bed  of  hard  limestone  amongst  more  yielding  strata, 
which  have  a  gentle  dip  upstream.  By  the  constant 
wash  of  the  water  the  soft  shales  underlying  the  lime- 
stone are  gradually  removed,  and  the  overlying  mass, 
losing  its  support,  breaks  away  from  time  to  time 
along  its  joint-planes.  In  this  manner  the  Falls  have 
slowly  retreated  from  Queenstown,  and  the  gorge  of 


CLASS TFICA  TION  OF  LAND-FORMS  355 

Niagara  has  been  formed.  The  Falls  of  Clyde  are 
due  to  a  precisely  similar  geological  structure,  and 
many  ravines  and  gorges  in  the  valleys  of  our  low- 
lands have  originated  in  the  same  way  as  the  gorge 
of  Niagara. 

The  occurrence  of  great  waterfalls  in  a  long-estab- 
lished hydrographic  system  is  somewhat  anomalous, 
and  leads  at  once  to  the  suspicion  that  the  drainage- 
system  has  been  interfered  with.  Waterfalls  cannot 
be  of  any  great  age.  Sooner  or  later  they  must  be 
cut  back  and  replaced  by  ravines  or  gorges.  Their 
presence,  therefore,  shows  either  that  the  valleys  in 
which  they  occur  are  throughout  of  recent  age,  and 
that  the  rivers  have  not  yet  had  time  to  reduce  such 
irregularities,  or  that  the  drainage-system,  if  long 
established,  has  since  been  disturbed  by  some  other 
agent  than  running  water.  In  deformation-mount- 
ains of  recent  age  we  naturally  expect  to  meet  with 
cascades  and  waterfalls,  for  the  streams  and  rivers  of 
such  a  region  are  relatively  young.  They  have  only, 
as  it  were,  commenced  the  work  of  erosion.  But 
plains  and  plateaux  of  erosion  which  have  existed  for 
ages  as  dry  land,  and  in  which  a  complete  hydro- 
graphic  system  has  been  long  established,  should 
show  no  great  waterfalls.  Yet  we  find  cascades  and 
waterfalls  more  or  less  abundantly  developed  in  all 
the  plains  and  plateaux  of  Northern  Europe  and  the 
corresponding  latitudes  of  North  America;  and  most 
of  these  lands  are  of  very  great  antiquity,  their  main 
lines  of  drainage  having  been  established  for  a  long 


356  EARTH  SCULPTURE 

time.  Obviously  the  hydrographic  systems  have 
been  disturbed,  and  the  disturbing  element  has  been 
glacial  action.  During  the  Ice  Age  the  long-estab- 
lished preglacial  contours  were  greatly  modified. 
Frequently,  indeed,  the  minor  valleys  in  plateaux  and 
plains  were  completely  obliterated,  while  even  the 
main  valleys  were  often  choked  with  debris.  When 
glacial  conditions  passed  away,  and  streams  and  rivers 
again  flowed  over  the  land,  they  could  not  always 
follow  the  old  lines  of  drainage  continuously,  but 
were  again  and  again  compelled  to  leave  those  and 
to  cut  out  new  courses  in  whole  or  in  part.  Hence 
the  frequent  occurrence  of  cascades  and  waterfalls  in 
formerly  glaciated  lands. 

Another  cause  for  the  existence  of  waterfalls  in 
long-established  hydrographic  systems  must  be  sought 
for  in  crustal  disturbances.  In  general,  deformations 
of  the  crust  would  seem  to  have  been  very  gradually 
brought  about,  so  gradually,  indeed,  that  they  have 
often  had  little  or  no  influence  upon  the  courses  of 
great  rivers.  Anticlines  slowly  developing  across  a 
river-valley  have  been  sawn  through  by  the  river  as 
fast  as  they  arose.  Dislocations,  in  like  manner, 
would  seem  to  have  been  very  slowly  developed. 
Frequently  these  have  traversed  a  river-valley  with- 
out in  any  way  disturbing  the  drainage,  the  rate  of 
erosion  having  been  equal  to  that  of  the  displace- 
ment. On  the  other  hand,  we  know  that  faulting 
or  dislocation  may  sometimes  be  rather  suddenly 
effected.  Thus,  a  large  fault  crossing  a  river-valley 


CLASSIFICATION  OF  LAND-FORMS  357 

and  having  its  downthrow  in  the  direction  in  which 
the  river  is  flowing  would  certainly  produce  a  water- 
fall. Such,  indeed,  would  appear  to  be  the  origin  of 
the  great  falls  of  the  Zambesi. 

A  volume  might  be  written  on  the  many  appear- 
ances presented  by  subsequent  or  erosion  valleys,  but 
it  is  beyond  our  purpose  to  enter  into  further  details. 
It  is  enough  to  recognise  the  fact  that  the  great  ma- 
jority of  river-valleys  have  been  excavated  by  the 
rivers  themselves.  Even  the  most  recent  tectonic 
valleys  have  often  been  profoundly  modified  by  sub- 
sequent erosion.  In  all  regions,  whatever  their  char- 
acter may  be,  whether  plateaux  and  plains  of  erosion 
or  accumulation,  or  true  mountains  of  elevation,  the 
streams  and  rivers  are  constantly  striving  to  reduce 
the  land  to  their  base-level.  The  main  directions  or 
lines  of  erosion  are  early  established  ;  but  in  the  course 
of  time  many  modifications  arise,  owing  to  the  work 
of  the  streams  and  rivers  and  of  epigene  agents  gen- 
erally. At  first,  it  may  be,  the  rivers  descend  by  a 
succession  of  steps  or  by  alternate  steep  and  more 
gentle  declivities.  Cascades,  waterfalls,  and  rapids, 
and  here  and  there  barrier-lakes,  may  abound.  But 
eventually  the  irregularities  are  removed  and  a  true 
curve  of  erosion  is  produced.  Each  river  has  then  its 
relatively  short  torrent-track,  and  its  longer  valley- 
and  plain-tracks.  As  erosion  proceeds  the  plain-track 
continues  to  encroach  inland  upon  the  valley-track, 
while  the  latter  eats  back  into  the  torrent-track.  At 
the  same  time  the  entire  surface  of  the  land  is  being 


358  EARTH  SCULPTURE 

continuously  reduced,  until  at  last  hills  and  mount- 
ains gradually  disappear,  and  the  whole  region  is  re- 
placed by  a  plain.  The  cycle  of  erosion,  however,  is 
not  often  allowed  to  proceed  without  interruption. 
Sometimes  an  upward  movement  increases  the  gradi- 
ents, and  so  in  time  the  revived  rivers  deepen  their 
courses,  and  "  valley  within  valley  "  appears.  Or  the 
whole  region  may  become  subject  to  glaciation,  during 
which  the  preglacial  drainage-system  may  be  consid- 
erably modified  by  erosion  here  and  accumulation 
there.  When  at  last  the  ice-covering  vanishes,  lakes, 
rapids,  cascades,  and  waterfalls  diversify  the  water- 
courses. But  the  removal  of  these  features  is  only  a 
matter  of  time.  By  and  by  all  the  direct  effects  of 
glaciation  must  disappear.  Again,  long  before  a 
cycle  of  aqueous  erosion  is  completed  the  land  may  be 
submerged  and  more  or  less  deeply  covered  under 
new  accumulations.  Should  re-elevation  eventually 
ensue,  a  new  hydrographic  system  will  then  come 
into  existence,  but  this  may  not  coincide  in  any  part 
with  that  of  the  old  buried  land-surface. 

In  regions  where  soluble  rocks,  such  as  limestone, 
abound,  the  hydrographic  system  usually  presents 
strong  contrasts  to  those  we  have  just  been  consider- 
ing. Much  of  the  rainfall  finds  its  way  below  ground, 
where  a  complex  series  of  channels  is  gradually  licked 
out,  until  eventually  the  whole  drainage  may  become 
subterranean.  Usually,  however,  the  drainage  is 
partly  superficial  and  partly  underground,  the  rivers 
flowing  for  longer  or  shorter  distances  in  ordinary 


CLASSIFICATION  OF  LAND-FORMS  359 

valleys  of  erosion,  until  they  suddenly  plunge  below. 
Sometimes  they  emerge  from  their  subterranean 
courses  again  and  again  ;  at  other  times  they  never 
reappear  at  the  surface,  but  discharge  their  waters  on 
the  sea-floor.  Owing  to  the  frequent  collapse  of 
tunnels  and  caverns,  the  surface  of  a  calcareous  region 
is  apt  to  show  many  irregular  depressions,  and  the 
superficial  hydrographic  system  is  necessarily  very 
imperfectly  developed. 

5.  Basins.  The  large  and  small  depressions  of  the 
surface,  like  other  superficial  features,  have  been 
formed  in  various  ways.  Some  are  the  result  of  hypo- 
gene  action,  others  owe  their  origin  to  epigene  ac- 
tion, while  yet  others  are  due  to  both.  Not  a  few,  for 
example,  are  hollows  caused  by  deformation  of  the 
crust,  and  may  be  termed  tectonic  basins.  Some, 
again,  occupy  the  site  of  extinct  volcanoes,  or  are  due 
in  one  way  or  another  to  volcanic  action  ;  they  are 
our  volcanic  basins.  Depressions  caused  by  the  re- 
moval of  soluble  materials  from  below,  as  in  lime- 
stone countries,  may  be  called  dissolution  basins ; 
while  the  terms  alluvial  and  czolian  may  well  be  ap- 
plied to  all  basins  which  are  the  result  of  fluviatile 
and  aeolian  action  respectively.  Landslips,  etc.,  by 
obstructing  drainage  form  a  series  of  rock-fall  basins, 
while  glacial  action  is  responsible  for  a  large  class  of 
basins,  some  of  which  are  rock-basins,  others  barrier- 
basins,  and  yet  others  partake  of  the  character  of 
both.  All  these  are  termed  glacial  basins. 

Unless    they   are    very    capacious   and    extensive, 


360  EARTH  SCULPTURE 

basins  soon  become  obliterated.  Erosion  and  sedi- 
mentation are  too  active  to  permit  of  their  prolonged 
duration.  Exceptionally,  however,  tectonic  basins 
may  long  outlive  the  land-surface  upon  which  they 
first  appeared.  If  the  deformation  of  the  crust  to 
which  they  owe  their  origin  be  continued,  erosion 
and  sedimentation  may  be  unable  to  obliterate  them. 
Should  the  bed  of  a  great  lake  subside  at  approxi- 
mately the  same  rate  as  alluvial  matter  accumulates 
upon  it,  while  at  the  same  time  the  effluent  river  can- 
not succeed  in  draining  the  lake  dry,  it  is  obvious 
that  the  latter  may  endure  for  a  very  long  time. 
Sediments  reaching  a  thickness  of  many  thousands 
of  feet  might  come  to  be  deposited  in  such  a  lake, 
although  the  water  itself  had  never  been  more  than 
a  few  hundred  feet  in  depth.  The  lake  would  form 
the  base-level  for  all  the  surrounding  region,  the  sur- 
face of  which,  perhaps  mountainous  to  begin  with, 
would  be  gradually  lowered,  and  might  pass  through 
a  complete  cycle  of  erosion  before  the  lake  ceased 
to  exist.  In  a  word,  a  great  lake  or  inland  sea  may 
become  the  burial-place  of  the  high  grounds  that  sur- 
round it,  for  it  bears  the  same  relation  to  these  as  an 
ocean  to  a  continent. 

The  great  majority  of  lakes,  however,  do  not  oc- 
cupy tectonic  basins,  and  must  sooner  or  later  disap- 
pear. Even  tectonic  basins,  the  beds  of  which  have 
ceased  to  subside,  must  eventually  be  obliterated. 
As  a  matter  of  fact,  none  of  the  existing  lakes  of 
the  world  can  be  shown  to  be  of  great  geological 


CLASSIFICATION  OF  LAND-FORMS  361 

antiquity.  All  alike,  large  and  small,  are  of  recent 
age.  As  regards  their  geographical  distribution,  it 
is  singular  and  suggestive  that  they  appear  most 
abundantly  in  glaciated  lands,  in  mountains,  plateaux, 
and  lowlands  alike.  None  of  these  can  be  shown  to 
have  existed  before  the  Glacial  Period,  and,  with  few 
exceptions,  all  must  be  attributed  to  the  direct  and 
indirect  action  of  flowing  ice.  The  preglacial  hydro- 
graphic  systems  have  been  disturbed  mainly  by 
glacial  erosion  and  accumulation.  Many  of  the  larger 
basins,  however,  such  as  those  of  Lakes  Ladoga  and 
Onega  in  Europe,  and  Lakes  Superior,  Michigan, 
and  others  in  North  America,  are  probably  to  a  large 
extent  tectonic,  and  due  to  warping  or  deformation 
of  the  crust.  Not  a  few  of  the  smaller  lakes,  again, 
occupy  hollows  caused  by  the  irregular  accumulation 
of  fluviatile  sediments,  or  by  the  blocking  of  streams, 
etc.,  by  rock-falls  and  landslips,  while  here  and  there 
they  rest  in  depressions  produced  by  the  dissolution 
and  removal  of  soluble  materials.  Outside  of  the 
glaciated  areas  comparatively  few  lakes  of  any  kind 
exist,  and  the  most  important  of  these  occupy  tec- 
tonic and  volcanic  basins. 

6.  Coast-Lines.  Two  types  of  coast  may  be  distin- 
guished, namely,  regular  or  smooth,  and  irregular 
or  indented.  The  former  may  be  high  and  steep  or 
gently  shelving,  and  when  expressed  upon  a  map 
show  a  softly  undulating  or  sinuous  course.  The 
shape  assumed  by  the  coasts  themselves  is  naturally 
determined  by  the  nature  of  the  rock-masses  and 


362  EARTH  SCULPTURE 

their  geological  structure,  and  the  manner  in  which 
they  succumb  before  the  action  of  waves  and  breakers. 
The  coastal  configuration  is  likewise  influenced  in 
many  places  by  accumulation,  for  the  coast-line  is  not 
fixed,  but  continually  oscillates,  retreating  in  some 
places,  advancing  elsewhere.  Irregular  or  indented 
coast-lines  are  typically  represented  by  such  regions 
as  Norway.  Here  the  continuity  of  the  coast-line  is 
repeatedly  interrupted  by  long  inlets,  while  a  multi- 
tude of  islands  fringe  the  land.  Obviously,  the  trend 
of  such  a  coast-line  is  determined  by  the  configura- 
tion of  the  land  ;  the  long  inlets  and  fiords  are  merely 
the  submerged  lower  reaches  of  mountain-valleys. 
All  highly  indented  coasts,  indeed,  are  evidence  that 
the  land  is  either  sinking  now  or  has  recently  sunk. 

In  general,  it  may  be  said  that  the  average  trend 
and  configuration  of  the  coast-lines  of  the  globe  are 
determined  by  the  position  of  the  continents  in  rela- 
tion to  the  great  oceanic  depression.  The  former 
are  nowhere  co-extensive  with  what  is  known  as  the 
continental  plateau,  considerable  areas  of  which  are 
below  the  sea-level.  When  the  coast-lines  approach 
the  margin  of  that  plateau,  they  generally  continue 
for  long  distances  in  one  particular  direction,  are 
rarely  much  indented,  and  show  few  or  no  fringing 
islands.  Conversely,  when  they  recede  from  the 
edge  of  the  plateau,  their  trend  becomes  irregular, 
following  now  one  direction,  now  another,  numerous 
inlets  appear,  and  marginal  islands  usually  abound. 
Indented  or  irregular  coasts  are  not  the  result  of 


CLASSIFICATION  OF  LAND-FORMS  363 

marine  erosion.  Fiords,  rias,  and  other  indentations 
are  simply  submerged  valleys.  The  intricate  coast- 
lines of  North-west  Europe,  of  Greece  and  other 
parts  of  the  Mediterranean  lands,  of  Alaska,  and 
many  other  regions  have  been  determined  by  anteced- 
ent subaerial  erosion. 


CHAPTER  XVII 

CONCLUSION 

THE  STUDY  OF  THE  STRUCTURE  AND  FORMATION  OF  SURFACE- 
FEATURES  PRACTICALLY  INVOLVES  THAT  OF  THE  EVOLUTION 
OF  THE  LAND. 

IN  the  preceding  chapters  we  have  been  inquiring 
into  the  origin  of  surface-features,  and  have  come 
to  the  general  conclusion  that  these  cannot  be  ac- 
counted for  without  some  knowledge  of  geological 
structure.  We  have  learned  that  the  crust  of  the 
earth  has  experienced  many  changes — rocks  have 
been  tilted,  compressed,  folded,  fractured,  and  dis- 
placed. In  some  places  elevation,  in  other  places 
depression,  has  taken  place,  or  both  kinds  of  move- 
ment have  affected  the  same  area  at  different  times. 
The  crust  has  further  been  disturbed  in  many  regions 
by  vast  intrusions  of  molten  matter  ;  while  frequently 
volcanic  action  has  cumbered  the  surface  with  lava 
and  fragmental  ejecta.  It  might  seem,  therefore,  as 
if  the  varied  configuration  of  our  lands — mountain 
and  valley,  height  and  hollow — might  be  largely  if 
not  exclusively  due  to  subterranean  action.  But  the 
study  of  geological  structure  has  shown  us  that  enorm- 

364 


CONCLUSION  365 

ous  masses  of  material  have  been  removed  from  the 
land-surface,  and  that  however  much  that  surface 
may  have  been  influenced  by  crustal  disturbance,  yet 
its  varied  features,  as  a  rule,  owe  their  origin  directly 
to  denudation.  Great  mountain-chains  have,  indeed, 
been  upheaved  from  time  to  time,  fractures  and  dis- 
placements have  again  and  again  taken  place  ;  but 
even  the  youngest  mountains  have  been  so  modified 
by  the  various  epigene  agents  of  change  that  fre- 
quently their  original  configuration  has  been  almost 
completely  destroyed.  .  Earth  sculpture,  in  a  word, 
is  everywhere  conspicuous,  and  in  regions  which 
have  remained  for  long  ages  undisturbed  by  subter- 
ranean action  the  latter  has  had  only  an  indirect 
influence  in  determining  the  form  of  the  surface. 
All  the  great  ranges  of  tectonic  mountains  are  of 
relatively  recent  age.  Time  has  not  yet  sufficed  for 
their  complete  reduction.  On  the  other  hand,  the 
mountains  that  were  upheaved  in  the  earlier  stages 
of  the  world's  history  have  been  either  completely 
remodelled  or  entirely  demolished.  If  elevations 
still  often  mark  the  sites  of  the  chains  and  ranges  of 
Palaeozoic  times,  their  internal  geological  structure  yet 
shows  that  they  are  no  longer  tectonic  but  relict 
mountains.  In  short,  we  see  that  epigene  agents 
are  constantly  endeavouring  to  remove  the  irregular- 
ities which  result  from  crustal  disturbance.  Eleva- 
tions are  gradually  lowered,  and  sunken  areas  filled 
up.  But  the  process  of  levelling  the  land  is  not  in- 
frequently interrupted  by  renewed  crustal  movements. 


366  EARTH  SCULPTURE 

No  sooner,  however,  do  fresh  elevations  appear  than 
the  cycle  of  erosion  begins  again. 

"  The  hills  are  shadows,  and  they  flow 
From  form  to  form,  and  nothing  stands." 

Although,  as  a  rule,  it  is  not  hard  to  prove  that 
certain  surface-features  owe  their  origin  to  erosion,  it 
is  often  very  difficult,  or  even  impossible,  to  follow 
out  the  whole  process — to  trace  the  various  stages  in 
the  evolution  of  surface-features.  Pyramidal  mount- 
ains composed  of  horizontally  arranged  beds  are 
obviously  relict  mountains  ;  they  have  been  carved 
out  of  horizontal  strata.  That  much  anyone  can  see, 
and  for  the  student  of  physical  geography  it  is 
enough,  perhaps,  to  be  able  to  distinguish  such 
mountains  from  those  of  a  different  build.  But  a 
geologist  cannot  be  content  with  this  :  he  will  en- 
deavour to  trace  out  the  whole  history  of  the  process. 
He  will  ascertain,  if  he  can,  the  age  of  the  strata,  and 
the  conditions  under  which  they  were  accumulated, 
and  subsequently  elevated  and  eroded.  It  is  the 
story  of  the  evolution  or  development  of  the  land  and 
its  surface-features  that  he  will  strive  to  unfold.  In 
some  cases  the  evidence  is  so  simple,  full,  and  clear, 
that  its  meaning  can  hardly  escape  him.  More  fre- 
quently, however,  it  is  complicated,  incomplete,  and 
hard  to  read.  We  may  have  no  doubt  whatever  that 
the  various  surface-features  of  the  region  we  are  ex- 
amining owe  their  origin  to  denudation  ;  but  we  shall 
often  experience  great  difficulty  in  discovering  the 
successive  stages  through  which  the  land  must  have 


CONCLUSION  367 

passed  before  it  assumed  its  present  configuration. 
In  this  volume  we  have  confined  attention  very  much 
to  the  simple  part  of  the  subject,  and  have  tried  to 
show  what  kinds  of  features  are  due  to  hypogene  and 
epigene  action  respectively.  Incidentally,  however, 
reference  has  been  made  to  the  successive  geological 
changes  which  have  preceded  and  led  up  to  existing 
conditions.  It  is  almost  impossible,  indeed,  to  con- 
sider the  formation  of  surface-features  without  at  the 
same  time  inquiring  into  their  geological  history. 
And  not  infrequently  we  find  that  the  configuration 
of  a  land  is  the  outcome  of  a  highly  involved  series  of 
changes.  To  understand  the  distribution  of  its  hills 
and  valleys,  its  plains  and  plateaux,  and  the  whole 
adjustment  of  its  hydrographic  system,  we  may  have 
to  work  our  way  back  to  a  most  remote  geological 
period.  But  if  it  be  true  that  the  present  cannot  be 
understood  without  a  knowledge  of  the  past,  it  is  no 
less  true  that  physical  conditions  which  have  long 
passed  away  can  often  be  realised  in  the  existing 
arrangement  of  surface-features.  This  is  no  more  than 
might  have  been  expected  ;  for  if,  as  we  all  believe, 
there  has  been  a  continuity  in  geological  history,  the 
germ  of  the  present  must  be  found  in  the  past,  just  as 
the  past  must  be  revealed  in  the  present,  if  only  we 
have  skill  to  read  the  record.  Evolution,  in  a  word, 
is  not  less  true  of  the  land  and  its  features  than  of 
the  multitudinous  tribes  of  plants  and  animals  that 
clothe  and  people  it. 

This  fascinating  branch  of  geology  has  been  fol- 


368  EARTH  SCULPTURE 

lowed  with  much  assiduity  by  many  workers  in  many 
lands.  But  it  is  still  in  its  infancy,  and  much  remains 
to  be  accomplished.  We  have  all  learned  the  lesson 
of  denudation.  We  know  that  rivers  have  excavated 
valleys,  that  the  whole  land-surface  is  being  gradually 
lowered  by  the  activity  of  the  epigene  agents.  But 
comparatively  few  have  set  themselves  the  task  of 
working  out  in  all  its  details  the  history  or  evolution 
of  the  varied  configuration  of  particular  areas.  Yet 
who  can  look  at  the  map  of  a  well-watered  region,  a 
land  of  mountain  and  glen,  of  rolling  lowlands  and 
countless  valleys,  without  a  wish  to  trace  out  the  devel- 
opment of  its  numberless  heights  and  hollows  ?  What 
a  world  of  interest  must  often  be  concentrated  in  the 
history  of  a  single  river  and  its  affluents  !  At  what 
time  and  under  what  conditions  did  it  first  begin  to 
flow  ?  How  was  its  course  and  those  of  its  tributa- 
ries determined  ?  Has  the  hydrographic  system  ever 
been  disturbed  ?  and  if  so,  to  what  extent  and  in  what 
manner  has  it  been  modified?  These  and  many 
similar  questions  will  come  before  the  investigator, 
and  in  searching  for  answers  he  must  often  unfold  a 
strange  and  almost  romantic  history. 

Naturally  investigation  of  the  kind  leads  up  to  the 
larger  inquiry — When  and  how  has  the  land  itself 
been  developed  ?  It  is  matter  of  common  knowledge 
that  the  lands  within  a  common  area  are  of  very 
different  age.  Some  have  only  recently  appeared ; 
others  are  of  vast  antiquity.  And  the  older  ones  can 
always  be  recognised  by  the  extent  to  which  earth- 


CONCLUSION  369 

sculpture  has  been  carried  on.  It  is  obvious,  there- 
fore, that  a  knowledge  of  the  features  produced  by 
erosion,  apart  from  other  geological  evidence,  must 
often  help  us  to  determine  the  relative  antiquity  of 
land-surfaces.  We  do  not  doubt  that  when  the  his- 
tory of  the  hydrographic  systems  of  the  continents 
has  been  better  worked  out,  when  the  evolution  of 
surface-features  has  been  more  closely  followed,  our 
knowledge  of  land-development  will  acquire  a  pre- 
cision to  which  it  cannot  at  present  lay  claim.  Geolo- 
gists will  then  also  be  better  prepared  to  attack  and 
perhaps  to  solve  the  largest  problem  of  all — the 
origin  of  our  continental  areas  and  oceanic  basins. 
Not  that  we  can  expect  or  desire  that  students  of 
nature  should  refrain  from  theorising  and  speculating 
in  that  direction  until  the  fuller  knowledge  we  de- 
siderate has  been  acquired.  Theory  must  often  be 
in  advance  of  the  evidence.  It  may  be  that  we  are 
already  in  possession  of  the  truth — that  the  con- 
tinental plateau  and  the  oceanic  depression,  as  many 
maintain,  are  primeval  wrinkles  of  the  crust.  At 
present,  however,  this  view  can  only  be  considered 
probable,  or,  as  some  would  say,  possible — a  brilliant 
suggestion  which  seems  to  explain  much  that  is 
otherwise  unintelligible. 

Another  question  that  will  obtrude  itself  when  we 
are  investigating  the  origin  of  surface-features  is  that 
of  time.  Surely  a  very  long  period  would  be  re- 
quired for  the  completion  of  a  cycle  of  erosion,  for 
the  upheaval  of  a  great  mountain-chain  and  its  subse- 


370  EARTH  SCULPTURE 

quent  resolution  to  a  plain  of  erosion,  for  the  cutting- 
up  of  a  lofty  plateau  into  hill  and  valley,  and  its  final 
complete  degradation.  We  find  it  difficult  to  conceive 
the  lapse  of  time  involved  "in  the  process,  and  the 
difficulty  is  increased  when  we  remember  that  cycles 
of  erosion  have  frequently  been  interrupted  by  long 
pauses,  during  which  the  regions  involved  have  been 
submerged,  and  not  only  protected  from  denudation, 
but  more  or  less  deeply  buried  under  new  accumula- 
tions. Yes,  assuredly,  we  must  admit  that  many  long 
ages  have  passed  since  the  process  of  land-sculpture 
began.  But  physicists  tell  us  that  we  can  no  longer 
draw  unlimited  drafts  upon  the  Bank  of  Time.  We 
have  no  immeasurable  and  countless  aeons  to  fall  back 
upon.  Moreover,  various  estimates  of  the  rate  at 
which  denudation  is  now  being  carried  on,  based  as 
these  are  on  the  amount  of  materials  carried  seawards 
by  rivers,  have  demonstrated  that  the  demand  for 
unlimited  time  is  not  justified.  Even  under  existing" 
moderate  climatic  conditions  our  own  land  is  being- 
levelled  at  a  rate  that  will  ensure  its  ultimate  degrad- 
ation within  a  period  not  so  infinitely  remote  as 
geologists  formerly  supposed.  In  short,  the  cumula- 
tive effect  of  small  changes  is  much  greater  than  was 
at  first  realised.  Further,  their  study  of  the  past  has 
taught  geologists  that  the  climate  of  the  world  has 
changed  from  time  to  time.  And  if  so,  then  the 
rate  of  denudation  must  likewise  have  varied.  In 
our  own  temperate  lands  we  see  how  slowly  erosion 
is  effected — our  streams  and  rivers  are  but  seldom 
clouded  with  much  sediment.  Even  after  the  lapse 


CONCLUSION  371 

of  many  years  their  courses  remain  apparently  un- 
modified. In  less  temperate  lands,  however,  erosion 
often  proceeds  apace  ;  watercourses  are  deepened 
and  widened  in  an  incredibly  short  time.  During 
a  tropical  storm  of  rain  as  much  erosion  of  soil  and 
rock  and  transport  of  material  are  effected  within  a 
limited  drainage-area  as  would  tax  a  British  river 
with  all  its  tributaries  to  accomplish  in  a  year  or  a 
number  of  years.  Now  these  islands  of  ours  have 
experienced  many  vicissitudes — tropical,  subtropical, 
and  arctic  conditions  have  formerly  obtained  here — 
and  we  need  not  doubt,  therefore,  that  the  present 
rate  of  denudation  has  often  been  exceeded  in  the 
past.  When  streams  and  rivers  began  their  work  of 
erosion  in  the  British  area,  it  is  probable  that  the 
climatic  conditions  were  more  favourable  for  that 
work  than  is  now  the  case.  In  a  word,  although  the 
work  performed  by  geological  agents  of  change  has 
been  the  same  in  kind,  it  has  necessarily  varied  in 
degree  from  time  to  time.  The  present  rate  of  ero~ 
sion  in  Britain,  therefore,  can  be  no  infallible  index 
to  that  of  the  past.  But  however  rapidly  denudation 
may  have  proceeded  in  former  ages,  the  shaping  out 
of  our  hills  and  valleys,  even  under  the  most  favour- 
able conditions,  must  have  been  a  slow  process. 
Nevertheless  recent  investigations  leave  little  room 
for  doubting  that  the  time  required  for  the  evolution 
of  all  the  multitudinous  forms  assumed  by  the  land 
has  been  exaggerated.  The  tale  told  by  our  relict 
mountains  and  erosion  valleys  does  not  support  the 
claim  for  unnumbered  millions  of  years. 


APPENDIX 

TABLE   OF    GEOLOGICAL    SYSTEMS,   AND    THEIR   PRINCIPAL 

SUBDIVISIONS. 


POST-TERTIARY    (  Pleistocene. 

f  Pliocene. 
TERTIARY  OR 
CAINOZOIC 


I   Miocene. 

I  Oligocene. 

Eocene. 


SECONDARY  OR 
MESOZOIC 


Cretaceous. 

Danian  (not  represented  in  England). 

Senonian  (Upper  Chalk  with  Flints). 

Turonian  (Middle  Chalk). 

Cenomanian  (Lower  Chalk  and  Upper  Greensand). 

Albian  (Gault). 

Aptian. 

r  /j_iuvvci     \jricci 

beds). 


'  /  /Lower  Greensand   and    Wealden 
Urgonian  .  > v 

Neocomian  ) 


White  Jura  or  Malm  of  Germany. 


.   )   Brown  Jura  or  Dog- 
36lite)  ) 


Jurassic. 

Purbeckian  . 

Portlandian . 

Kimeridgian 

Corallian 

Oxfordian    . 

Bathonian 

Bajocian  (Inferior  O&l 

Toarcian  (Upper  Lias) . 

Liasian  (Middle  and  Lower  Lias 

in  part) 

Sinemurian( Lower  Lias  in  part) 
Hettangian  (Infra-Lias) 

Triassic. 

Rhsetic. 

Keuper. 

Muschelkalk  (not  represented  in  England). 

Bunter. 


ger  of  Germany. 


Black  Jura  or 

Lias  of 

Germany. 


373 


374 


APPENDIX 


PRIMARY  OR 
PALAEOZOIC 


Permian. 

Zechstein  (Magnesian  Limestone  and  Marl  Slate). 

Rothliegendes  (Red  Sandstones,  Conglomerates, 

and  Breccias). 
Carboniferous. 

Coal  Measures. 

Millstone  Grit. 

Carboniferous  Limestone  Series. 
Devonian  and  Old  Red  Sandstone. 

Upper.      r»i  i  T?^J  (  Upper. 

_.          .  i   ... , ,,        vjld  Ked 

Devonian  .  •{   Middle.  •{ 

Sandstone.   /  T  »,,,__ 

Lower.  I  Lower. 


Silurian. 

Upper. 

Lower. 
Cambrian. 

Upper. 

Middle. 

Lower. 
Pre-Cambrian  or  Archaean. 


[NOTE. — The  names  of  the  subdivisions  of  the  various  systems  given  in  this 
table  are  those  generally  accepted.  Many,  it  will  be  seen,  are  of  English 
origin  ;  others  are  foreign.  Beside  some  of  the  latter  the  English  equivalents 
(which  are  still  current)  are  placed  within  parenthesis.  A  few  German  equiva- 
lents are  given  because  reference  is  made  to  them  in  the  text.] 


GLOSSARY 

Abrasion :  the  operation  of  wearing  away  by  aqueous  or  glacial  action. 

Acid  igneous  rocks :  rocks  which  contain  a  large  percentage  of  silica  to  a 
small  percentage  of  bases. 

Agglomerate :  volcanic  fragmental  rock,  consisting  of  large  angular,  sub- 
angular,  and  roughly  rounded  blocks,  confusedly  huddled  together. 

Alluvium  :  a  deposit  resulting  from  the  action  of  rivers  or  of  tidal  currents. 

Amygdaloidal  (Gr.  amygdalon,  an  almond  ;  eidos^  an  appearance)  :  applied  to 
igneous  rocks  containing  vesicular  cavities  which  have  become  filled,  or 
partially  filled,  with  subsequently  introduced  minerals.  The  cavities  are 
frequently  almond-shaped  ;  the  mineral  kernels  are  termed  amygdules. 

Anticline  (Gr.  a«//,  against  ;  klino,  I  lean) :  a  geological  structure  in  which 
strata  are  inclined  in  opposite  directions  from  a  common  axis  ;  i.  e.,  in  a 
roof-like  form.  When  its  axis  is  vertical,  an  anticline  is  symmetrical /  in 
an  unsymmetriccl  anticline  the  axis  is  inclined. 

Archaean  :  synonymous  with  Pre-Cambrian.     See  Table  of  Geological  Systems. 

Arenaceous  :  applied  to  strata  which  are  largely  or  wholly  composed  of  sand. 

Argillaceous  :  applied  to  rocks  composed  of  clay,  or  in  which  a  notable  pro- 
portion of  clay  is  present. 

Ash,  volcanic  :  the  finest-grained  materials  ejected  during  volcanic  eruptions. 

Basalt  :  a  dark,  hemicrystalline,  basic  igneous  rock. 

Base-level  of  Erosion:  that  level  to  which  all  lands  tend  to  be  reduced  by 

denudation.     A  land  base-levelled  is  usually  very  slightly  above  the  sea-level, 

and  shows  a  gently  undulating  or  approximately  flat  surface. 
Basic  igneous  rocks :  rocks  which  contain  a  large  percentage  of  bases  to  a 

low  percentage  of  silicic  acid. 
Beaches,  raised  :  former  sea-margins  ;  sometimes  appear  as  terraces  of  gravel, 

sand,  etc.,  sometimes  as  shelves  cut  in  solid  rock  ;  occur  at  all  levels,  from 

a  few  feet  up  to  several  hundred  yards  above  the  sea. 

375 


376  GLOSSARY 

Biotite  (Biot,  French  physicist)  :  a  black  or  dark-green  mica  ;  occurs  as  a  con- 
stituent of  many  crystalline  igneous  and  schistose  rocks. 

Bombs,  volcanic  :  clots  of  molten  lava  shot  into  the  air  from  a  volcano  ;  hav- 
ing a  rotatory  motion,  they  acquire  circular  or  elliptical  forms,  and  are  often 
vesicular  internally,  or  hollow. 

Bosses  :  large  amorphous  masses  of  crystalline  igneous  rock  which  have  cooled 
and  consolidated  at  some  depth  from  the  surface,  and  are  now  exposed  by 
denudation. 

Boulder-clay  :  typically,  an  unstratified  clay  more  or  less  abundantly  charged 
with  angular  and  subangular  stones  of  all  shapes  and  sizes  up  to  large 
blocks  ;  the  bottom  or  ground-moraine  of  prehistoric  glaciers  and  ice-sheets. 

Bunter  (Ger.  bunt,  variegated) :  one  of  the  subdivisions  of  the  Triassic  system  ; 
the  sandstones  of  the  Bunter  are  often  spotted  or  mottled. 

Buttes  (Fr.)  and  mesas  (Sp.) :  names  given,  in  the  Territories  of  the  United 
States,  to  conspicuous  and  more  or  less  isolated  hills  and  mountains. 
Bultes  are  usually  craggy,  precipitous,  and  irregular  in  outline  ;  mesas  are 
flat-topped  or  tabular. 

Cainozoic  (Gr.   kainos,  recent ;  zoe,  life).     See  Table  of  Geological  Systems. 
Calciferous  :  applied  to  strata  which  contain  carbonate  of  lime  as  a  binding  or 

cementing  material  ;  or  to  strata  among  which  numerous  beds  of  limestone, 

or  other  calcareous  rocks,  occur. 
Calc-sinter  (Ger.  kalk  (calx),  lime  ;  sinter,  a  stalactite) :  a  deposit  from  water 

holding  carbonate  of  lime  in  solution. 
Cambrian  (Cambria  or  Wales) :  name  given  by  Professor  Sedgwick  to  one  of 

the  Palaeozoic  systems  which  was  first  carefully  studied  in  Wales. 
Carboniferous  :  name  given  to  the  great  coal-bearing  system  of  the  Palaeozoic 

rocks. 
Chalybeate  (L.  chalybs,  steel) :  applied  to  water  impregnated  with  oxide  of 

iron. 
Chlorite  (L.  chloritis) :  a  greenish  mineral  present  in  some  schistose  rocks  ;  often 

occurs  in  igneous  rocks  as  a  product  of  alteration. 

Clastic  (Gr.  klastos,  broken):  applied  to  rocks  composed  of  fragmental  materials. 
Clinkers  (Dut.  klinker,  that  which  sounds)  :  the  cindery-like  masses  forming 

the  crust  of  some  kinds  of  lava. 
Concretion  :  a  body  formed  by  irregular  aggregation  or  accretion  of  mineral 

matter,  very  often  round  a  nucleus  ;  may  be  spherical,  elliptical,  or  quite  ir- 
regular and  amorphous.    Concretionary,  formed  of  or  containing  concretions. 
Coulee  (F.) :  a  stream  of  lava,  whether  flowing  or  become  solid. 


GLOSSARY 


377 


Crag-and-tail  :  a  hill  or  crag  showing  an  abrupt  and  often  precipitous  face  on 
one  side,  and  sloping  away  gradually  to  the  low  ground  in  the  opposite 
direction. 

Cretaceous  :  name  given  to  the  great  chalk-bearing  system  of  the  Mesozoic 
strata. 

Crust  of  the  Earth :  the  outer  portion  of  the  earth  which  is  accessible  to 
geological  investigation. 

Curve  of  Erosion  :  A  typical  river  has  its  steep  mountain-track,  its  moderate 
valley-track,  and  its  gentle  plain-track.  In  the  case  of  young  rivers,  the 
change  from  the  one  track  to  the  other  is  often  abrupt.  In  older  river- 
courses,  such  irregularities  tend  to  be  more  and  more  reduced — the  transi- 
tion from  the  one  track  to  the  other  becomes  gradual — until  eventually  the 
course  may  be  represented  by  a  single  curve,  flattening  out  as  it  descends 
from  source  to  mouth.  This  is  the  curve  of  erosion. 


Debacle  (F.) :  a  tumultuous  rush  of  water,  sweeping  forward  rock  debris,  etc. 

Deflation  :  the  denuding  and  transporting  action  of  the  wind. 

Degradation  :  the  wasting  or  wearing  down  of  the  land  by  epigene  agents. 

Denudation  :  the  laying  bare  of  underlying  rocks  by  the  removal  of  superficial 
matter  ;  the  process  by  which  the  earth's  surface  is  broken  up  and  the  ma- 
terials carried  away. 

Derivative  rocks  :  rocks  which  have  been  formed  out  of  the  materials  of  pre- 
existing minerals,  rocks,  and  organic  remains. 

Detritus  :  any  accumulation  of  materials  formed  by  the  breaking-up  and  wear- 
ing-away  of  minerals  and  rocks. 

Devonian  :  name  given  to  one  of  the  Palaeozoic  systems  ;  it  is  well  developed  in 
Devonshire. 

Diluvium  :  name  given  to  all  coarse  superficial  accumulations  which  were  for. 
merly  supposed  to  have  resulted  from  a  general  deluge  ;  now  employed  as  a 
general  term  for  all  the  glacial  and  fluvio-glacial  deposits  of  the  Ice  Age. 

Diorite  (Gr.  dioros,  a  boundary  between) :  a  crystalline  igneous  rock,  belonging 
to  a  group  intermediate  in  composition  between  the  basic  and  acid  groups. 

Dogger :  one  of  the  subdivisions  of  the  Jurassic  system  in  Germany,  etc. 
Dolerite  (Gr.  doleros,  deceptive) :  a  crystalline  basic  igneous  rock. 

Dolina  (It.) :  name  given  to  the  funnel-shaped  cavities  which  communicate 
with  the  underground  drainage-system  in  limestone  regions.  Similar  cavi- 
ties are  known  in  this  country  'as  sinks  and  swallow-holes. 


378  GLOSSARY 

Dolomite  (Dolomieu,  the  French  geologist) :  carbonate  of  calcium  and  magne- 
sium ;  occurs  as  a  crystallised  mineral,  and  also  as  a  granular  crystalline 
rock  (magnesian  limestone). 

/Drum,  Drumlin  (Ir.  and  Gael,  druman,  the  back,  a  ridge) :  a  ridge  or  bank  of 
boulder-clay  alone,  or  of  "  rock  "  and  boulder-clay.  Ridges  of  this  kind 
often  occur  numerously.  There  seem  to  be  two  varieties — (a)  long  parallel 
ridges  or  banks,  and  (b)  short  lenticular  hillocks  ;  the  former  usually  consist 
of  glacial  accumulations  alone  ;  the  latter  not  infrequently  contain  a  core  or 
nucleus  of  solid  rock,  or  they  may  show  solid  rock  at  one  end  and  glacial 
materials  at  the  other. 

Dyas  (LL.  the  number  two) :  name  sometimes  applied  to  the  Permian  system 
with  reference  to  its  subdivision  into  two  principal  groups. 

Eocene  (Gr.  tos,  dawn  ;  kainos,  recent)  :  see  Table  of  Geological  Systems. 
•^Epigene  (Gr.  epi,  upon  ;   gennao,  I  produce) :    applied  to  the  action  of  all  the 
geological  agents  of   change  operating  at  or  upon  the  earth's  surface  ;  also 
to  all  accumulations  formed  by  the  action  of  those  agents. 

Erratics  :  boulders  and  fragments  of  rock  which  have  been  transported,  gener- 
ally by  the  agency  of  glaciers  or  floating  ice,  and  are  therefore  foreign  to 
the  places  in  which  they  occur. 

Eruptive  rocks  :  massive  igneous  rocks  generally  ;  properly  only  those  which 
have  been  extruded  at  the  surface  are  truly  eruptive  ;  molten  masses  which 
have  been  intruded  in  the  crust,  and  therefore  below  the  surface,  are  ir- 
ruptive. 

^  Eskers  (Ir.  eiscir,  a  ridge) :  ridges  of  gravel  and  sand  which  appear  to  have 
been  formed  in  tunnels  underneath  the  great  glaciers  and  ice-sheets  of 
former  times  ;  same  as  the  Swedish  osar. 


Pelspars  (Ger.  feld,  a  field  ;  spath,  spar) :  a  group  of  minerals,  common  con- 
stituents of  many  igneous  and  schistose  rocks. 

Fire-clay  :  properly  a  clay  suitable  for  the  manufacture  of  fire-bricks  ;  in  geo- 
logy is  applied  to  the  argillaceous  layer  underlying  most  coal-seams,  which 
consists  generally  of  some  kind  of  clay,  but  is  not  always  suitable  for  fire- 
bricks. 

Fluvio-glacial :  applied  to  sedimentary  deposits  resulting  from  the  action  of 
streams  and  rivers  escaping  from  a  glacier  or  an  ice-sheet. 

Foliated  rocks  :  another  name  for  schist  and  schistose  rocks.     See  Schist. 

Formation :  a  series  of  rocks  having  some  character  in  common,  whether  of 
origin,  age,  or  composition  ;  often  applied  to  a  group  of  strata  containing  a 


GLOSSARY  379 

well-marked  and  distinctive  assemblage  of  fossils — a  group  of  subordinate 
importance  to  a  system. 
Fragmental  rocks  :  see  Clastic  and  Derivative. 

Gabbro  (It.) :  a  coarsely  crystalline  basic  igneous  rock. 

Geanticline  (Gr.  gt,  the  earth  ;  and  F.  anticline) :  a  broad  or  regional  arching 
or  bending  up  of  the  crust — thus,  a  geanticline  may  be  composed  of  strata 
showing  all  kinds  of  geological  structure.  It  is  simply  a  bulging  or  swelling 
up  of  the  crust  which  affects  a  wide  region.  Gtosyncline  is  just  the  oppo- 
site :  it  is  a  wide  or  broad  region  of  depression,  i.  t.,  a  sinking  of  the  earth's 
crust  as  a  whole. 

Geysers  (Icel.) :  eruptive  fountains  of  hot  water  and  steam. 

Giants'  kettles  :  large  pot-holes  often  observed  in  the  deserted  beds  of  old 
glaciers  ;  they  are  believed  to  have  been  drilled  by  water  descending  from 
the  surface  of  the  glaciers  and  setting  stones  and  boulders  in  rapid  rota- 
tion. 

Glacial  Period  :  the  deposits  of  the  Ice  Age  referred  to  in  the  text  belong  for 
the  most  part  to  the  Pleistocene  system.  Cold  climatic  conditions,  however, 
had  set  in  before  the  close  of  the  Pliocene,  and  were  continued  into  the  Re- 
cent period — the  last  of  our  snow-fields  and  glaciers  having  vanished  during 
the  formation  of  some  of  the  youngest  raised  beaches — a  time  when  Neo- 
lithic man  lived  in  Britain. 

Gneiss  (Ger.);  one  of  the  more  coarsely  crystalline  schistose  rocks. 

Granite  (It.  granito) :  one  of  the  deep-seated  plutonic  crystalline  igneous  rocks. 
Granitoid,  having  the  structure  of  granite. 

Greywacke  (Ger.  grauwackJ]  :  a  sedimentary  rock,  somewhat  metamorphosed; 
common  in  the  Palaeozoic  systems. 

Grit :  generally  a  coarse-grained  arenaceous  rock ;  the  harder  kinds  are  used 
for  grindstones. 

Ground-moraine  :  the  rock-rubbish  formed  by  the  grinding  action  of  glaciers 
and  ice-sheets. 

Gypsum  (Gr.  gypsos,  chalk) :  a  crystalline  mineral  composed  of  sulphate  of 
lime. 

Hade :  a  miner's  term  for  the  inclination  or  deviation  of  a  lode  or  fault  from 

the  vertical. 
Haematite  (Gr.   haimatites,  blood-like):    a  mineral   compound  of  oxide  of 

iron,  which  yields  a  blood-red  streak  when  scratched. 
Holocrystalline  (Gr.  holos,  whole  ;  F.  crystalline)  :  applied  to  igneous  rocks 

composed  entirely  of  crystalline  ingredients,  as  granite. 


380  GLOSSARY 

Hornblende  (Ger.  horn,  horn  ;  blenden,  to  dazzle) :  a  mineral  constituent  of 
many  crystalline  igneous  and  schistose  rocks. 

Horste  :  name  given  by  German  geologists  to  isolated  mountains  severed  by 
dislocations  from  rock-masses  with  which  they  were  formerly  continuous, 
but  which  have  since  subsided  to  a  lower  level.  Kumpfgebirge  (lit.,  rump- 
mountains)  is  another  name  for  this  type  of  mountain. 

Humous  acids :  general  name  for  the  various  acids  met  with  in  the  humus  or 
vegetable  mould,  and  which  are  derived  from  the  decomposition  of  organic 
matter. 

^  Hypogene  (Gr.  hypo,  under  ;  gennao,  I  produce) :  applied  to  geological  action 
under  the  earth's  surface,  and  to  the  products  of  that  action  ;  opposed  to 
Epigene  (q.  z>.). 

Infraglacial :  applied  to  deposits  formed  and  accumulated  underneath,  or  in 

the  bottom  parts  of,  glaciers  and  ice-sheets  ;  and  to  the  geological  action 

of  the  ice  upon  rocks  over  which  it  flows. 
In  situ  :  in  its  original  situation  ;  applied  to  minerals,  fossils,  and  rocks  which 

occupy  their  natural  place  or  position. 

Insolation  :  the  geological  action  of  the  sun's  heat  upon  rocks  at  the  surface. 
Intraglacial :  applied  to  rock-fragments  embedded  in  the  central  and  upper 

portions  of  glaciers  and  ice-sheets. 
I     Intrusive  rocks :  molten  rocks  which  have  been  injected  among  pre-existing 

rock-masses. 
Inversion :  a  geological  structure  in  which  strata  have  been  so  folded  as  to  be 

turned  upside  down. 
I    Isoclinal  (Gr.  isos,  equal  ;  klino,  to  lean) :  applied  to  strata  folded  in  a  series 

of  unsymmetrical  anticlines  and  synclines  whose  axes  all  incline  in  one  and 

the  same  direction. 

Joints :  natural  division-planes  which  intersect  bedded  and  amorphous  rocks 
of  all  kinds.  In  bedded  rocks  two  sets  of  joints  are  usually  recognisable 
(master -joints),  which  cut  each  other  at  approximately  right  angles.  In 
crystalline  igneous  and  schistose  rocks  the  joints  as  a  rule  are  somewhat 
irregular  ;  but  to  this  there  are  exceptions — as  in  certain  granites,  basalts, 
etc. — many  of  the  fine-grained  igneous  rocks  showing  prismatic  jointing  or 
columnar  structure. 

Jurassic  (from  Jura  Mountains)  :  one  of  the  Mesozoic  systems. 

,,  Kames  :  ridges  and  mounds  of  gravel  and  sand  generally,  but  now  and  again 
of  rude  rock-rubbish.  They  are  of  glacial  and  fluvio-glacial  origin,  having 
been  accumulated,  in  many  cases,  along  the  terminal  margins  of  large 
glaciers  and  ice-sheets. 


GLOSSARY  381 

Kaolin  (Chin,  kaoling)  \  a  fine  clay  resulting  from  the  chemical  decomposition 
of  felspar. 

Keuper  (Ger.)  :  one  of  the  subdivisions  of  the  Triassic  system. 

Laccolith  (Gr.  lakkos,  a  cistern  ;  lithos,  stone) :  name  given  to  intrusive  rocks 
which,  when  rising  from  below,  have  spread  out  laterally,  so  as  to  form 
lenticular  masses,  thereby  lifting  the  rocks  above  them  so  as  to  form  dome- 
shaped  swellings  at  the  surface. 

Lapilli  (L.) :  small  stones  ejected  from  volcanoes  in  eruption. 

Lee-seite  (Ger.) :  the  side  of  a  hill  or  prominent  rock  in  a  glaciated  region 
which  has  been  sheltered  or  protected  by  its  position  from  the  abrading 
action  of  the  ice-flow.  The  opposite  side,  exposed  to  that  action,  and 
therefore  "  glaciated,"  is  termed  the  Stoss-seite. 

Lias  :  one  of  the  subdivisions  of  the  Jurassic  system. 

Lignite  :  brown  coal,  not  so  highly  mineralised  as  common  coal. 

Maars :  name  given  in  the  Eifel  to  crater-lakes. 

Macalubas  :  mud-volcanoes,  so  called  from  the  well-known  Macalube,  near 

Girgenti,  in  Sicily. 
Magma  :  the  molten  or   plastic  material  which,  when  cooled  and  solidified, 

forms  crystalline,  hemicrystalline,  or  glassy  igneous  rocks. 
Malm  :  one  of  the  subdivisions  of  the  Jurassic  system  in  Germany,  etc. 
Master-joint :  see  Joints. 
Mesa :  see  Buttes. 

Mesozoic  (Gr.  mesos,  middle  ;  zoe,  life)  :  see  Table  of  Geological  Systems. 
Metamorphic  (Gr.  meta,  expressing  change  ;  morphe,  form)  :  applied  to  rocks 

which  have  been  more  or  less  completely  changed  in  form  and  structure — 

their  constituent  materials  having  been  rearranged. 
Mica  :  a  group  of  minerals,  common  constituents  of  many  igneous  and  schistose 

rocks. 

Millstone  Grit  :  one  of  the  subdivisions  of  the  Carboniferous  system. 
Miocene  (Gr.  meion,  less  ;  kainos,  recent) :  one  of  the  Cainozoic  systems. 
Monocline  (Gr.   monos,  single  ;  klino^  to  lean)  :  the  simplest  kind  of  fold  ;  an 

abrupt  increase  of  dip  in  gently  inclined  or  approximately  horizontal  strata, 

followed  by  an  equally  abrupt  return  to  the  original  position. 
Moulin  (F.,  a  mill)  :  an  approximately  vertical  cavity  or  shaft  worked  out  in 

a  glacier  by  water  descending  from  the  surface  through  a  crevasse.     See 

Giants'  kettles. 


382  GLOSSAR  Y 

Necks  :  plugg@d-up  pipes  of  volcanic  eruption  ;  the  throats  of  old  volcanoes 

which  have  been  laid  bare  by  denudation. 
N£v6  (F.) :  granular  snow  ;  the  condition  assumed  by  snow  on  its  passage  into 

glacier-ice. 

Obsidian  :  a  volcanic  glassy  rock. 

Old  Red  Sandstone  :  see  Table  of  Geological  Systems. 

1  Oligocene  (Gr.  oligos,  little  ;  kainos,  recent)  :  one  of  the  Cainozoic  systems. 
Olivine  :  a  greenish  mineral  ;  a  common   constituent  of  many  basic  igneous 

rocks. 
Oolite  (Gr.  oon,  an  egg  ;  lithos,  stone)  :  a  granular  limestone,  comn.on  in  tr 

Jurassic  system,  which  on  this  accouut  used  to  be  known  as  the  O&litic 
/        formation. 

/  Osar  (Swedish) :  see  Eskers. 

Outlier :  a  detached  mass  of  rock  resting  upon  and  surrounded  on  all  sides  by 
older  rocks. 

Overfold  :  an  overturned  or  inverted  fold  ;  the  axis  so  inclined  that  one  limb  of 
the  fold  is  doubled  back  under  the  other.  When  the  axis  becomes  hori- 
zontal, or  nearly  so,  the  fold  is  recumbent. 

Overthrust :  a  faulted  overfold  ;  the  fold  has  been  dislocated,  and  one  limb 
pushed  over  the  other  along  a  thrust-plane. 

Palaeozoic  (Gr.  palaios,  ancient  ;  zoc,  life) :  see  Table  of  Geological  Systems. 

Parallel  roads  :  old  lake-beaches,  seen  in  Glen  Roy  (Scottish  Highlands)  and 
other  valleys  in  its  neighbourhood. 

Paysage  morainique  :  a  region  abundantly  covered  with  terminal  moraines. 

Perched  blocks :  boulders  transported  by  glacier-ice  and  stranded  in  pro- 
minent positions. 

/  Petrography  (Gr.  petros,  a  rock  ;  grapho,  to  describe) :  the  study  of  rocks — 
Petrology  and  Lithology. 

Phonolite  (Clinkstone)  (Gr.  phone,  sound  ;  lithos,  stone)  :  a  volcanic  crystalline\ 
rock  ;  when  fresh  and  compact  it  has  a  metallic  ring  or  clink  under  the 
hammer. 

1    Pleistocene  (Gr.  pleistos,  most ;   kainos,  recent)  :   one  of   the   Post-Tertiary 
systems. 

Pliocene  (Gr.  pleion,  more  ;  kainos,  recent) :  one  of  the  Cainozoic  systems. 

Plutonic  (Pluto,  the  god  of  the  infernal  regions) :  applied  to  deep-seated 
igneous  action  ;  also  to  deep-seated  igneous  rocks  —  those  which  have 
cooled  and  consolidated  at  some  depth  from  the  surface. 


GLOSSAR  Y  383 

'    Post-Tertiary  or  Quaternary  :  the  youngest  group  of  systems.     See  Table. 
Ix  Pre-Cambrian  or  Archaean  :  the  oldest  system  of  rocks. 
cJPumice  :  any  froth-like,  foam-like,  spongy,  porous,  or  cellular  lava. 
Pyroxene  (Gr.  pur,  fire  ;  xenos,  a  guest) :  a  family  of  minerals,  common  con- 
stituents of  many  crystalline  igneous,  and  of  some  schistose  rocks. 

Quadersandstein  :  name  given  in  Saxony,  Bohemia,  and  Silesia  to  the  Cre- 
taceous system  ;  so  called  because  the  sandstone  of  which  it  is  chiefly 
composed  is  traversed  by  abundant  well-marked  vertical  joints,  that  cause 
the  rock  to  weather  into  square,  tabular,  and  pyramidal  hills,  and  pillar- 
like  masses. 

Quaquaversal  (L.  quaqua,  wheresoever  ;  versus,  turned)  :  applied  to  strata 
which  dip  outwards  in  all  directions  from  a  common  centre  ;  dome-shaped 
strata. 

Quartz  (Ger.) :  common  form  of  native  silica  ;  the  most  common  of  all  rock- 
forming  minerals. 

Quaternary  :  alternative  name  for  Post-Tertiary. 

Raised  beaches  :  see  Beaches. 

Recent  period  :  the  latest  of  the  geological  systems  ;  passes  gradually  into  the 
present  or  existing  condition  of  the  earth. 

Reversed  faults  :  in  these  the  hade  or  inclination  of  the  fault  is  in  the  direc- 
tion of  upthrow — lower  rocks  having  been  pushed  over  higher  rocks.  See 
Overthrust  and  Thrust-plane. 

Revived  rivers :  when  the  rivers  of  a  region  have  succeeded  in  cutting  their 
channels  down  to  the  base-level,  they  have  a  slight  fall  and  flow  sluggishly. 
Should  the  whole  region  then  be  elevated,  while  the  direction  of  its  slopes 
remains  unchanged,  the  erosive  energy  of  the  rivers  is  renewed,  and  they 
are  said,  therefore,  to  be  revived. 

Rhaetic  (from  the  Rhaetian  Alps) :  one  of  the  subdivisions  of  the  Triassic  system. 

Rhyolite  (Gr.  rheo,  to  flow  ;  lithos,  stone) :  an  acid  volcanic  rock. 

Roches  moutonn6es  :  rocks  rounded  like  the  back  of  a  sheep  ;  name  given  to 
rocks  which  have  been  abraded,  rounded,  and  smoothed  by  glacial  action. 

Rothliegendes  (Ger.)  :  one  of  the  subdivisions  of  the  Permian  system. 

Rumpfgebirge  (Ger.):  same  as  Horste  (q.  v.). 

Salses  (Fr.) :  another  name  for  mud-volcanoes  or  Macalubas  (q.  v.). 
'-•   Schist  (Gr.  sehistos,  easily  split):  a  crystalline  rock  in  which  the  constituent 
minerals  are  arranged  in  rudely  alternate  parallel  layers  or  folia  ;  a  foliated 
rock. 


384  GLOSSARY 

Scoriae  (Gr.  skoria,  dross) :  loose  fragments  of  slaggy,  cindery  lava. 

Screes  (Icel.  skritha,  fallen  rocks  on  a  hillside) :  a  Westmoreland  term  for  the 
sheets  of  loose  angular  stones  which  gather  upon  hillsides  and  at  the  base  of 
cliffs,  etc. 

t  Shearing  :  the  yielding  of  a  rock  to  compression,  strain,  and  tension  during 
crustal  movements,  whereby  the  solid  mass  is  compelled  to  flow,  so  that  a 
kind  of  fluxion-structure  is  developed  in  it ;  frequently  under  such  condi- 
tions dislocation  takes  place — the  rock  gives  way  and  one  mass  is  pushed 
over  another. 

Sheet :  molten  matter  intruded  between  bedded  rocks. 

'  Stalactites  (Gr.  stalaktos,  dropping) :  the  icicle-like  pendants  hanging  from  the 
roofs  of  limestone  caves,  formed  by  the  drip  of  water  holding  carbonate  of 
lime  in  solution. 

Stalagmites  (Gr.  stalagmos,  a  dropping)  :  the  calcareous  deposit  formed  upon 
the  floor  of  a  cavern  by  the  drip  of  water  from  the  roof. 

Stoss-seite :  see  Lee-seite. 

Striae,  glacial :  scratches,  furrows,  etc.,  engraved  upon  rock-surfaces  by  glacial 
action. 

Strike  :  the  general  direction  or  run  of  the  outcrops  of  strata. 

Swallow-holes :  see  Dolina. 

Syenite  (from  Syene,  Egypt) :  a  holocrystalline  igneous  rock  of  deep-seated 
origin. 

Syncline  (Gr.  syn,  together  ;  klino^  I  lean)  :  a  basin  or  trough-shaped  arrange- 
ment of  strata  ;  the  strata  dip  from  opposite  directions  inwards  to  one 
common  axis.  When  the  axis  is  vertical  the  syncline  is  symmetrical ;  when 
inclined,  unsymmetrical. 

Systems  :  the  larger  divisions  of  strata  included  under  the  Palaeozoic,  Mesozoic, 
Cainozoic,  and  Quaternary  groups. 

Terrigenous :  applied  to  marine  accumulations  the  materials  of  which  have 
been  derived  from  land  ;  opposed  to  abysmal,  applied  to  marine  deposits  the 
constituents  of  which  have  not  been  so  derived. 

^/Thrust-plane :  a  Reversed  fault  (q.  z>.),  the  hade  or  inclination  of  which 
approaches  horizontality  ;  a  common  structure  in  regions  of  highly  flexed 
rocks. 

Till :  another  name  (Scottish)  for  Boulder-clay  (q.  v.}. 

Tors  :  the  peculiar  and  often  fantastic  prominences  met  with  in  regions  of 
granite  which  have  been  long  exposed  to  weathering,  as  on  Dartmoor.  The 
kopjes  of  Mashonaland  are  an  example  of  the  same  phenomenon. 


GLOSSARY  385 

Trachyte  (Gr.  trachys,  rough) :  a  hemicrystalline  volcanic  rock. 

Travertine  :  another  name  for  Calc-sinter  (q.  v.). 

Triassic  (Gr.  trias,  three)  :  one  of  the  Mesozoic  systems. 

Tufa,  or  calcareous  tufa:  same  as  Calc-sinter,  Travertine  (q.  ».). 

Tuff:  a  volcanic  fragmental  rock;  usually  applied  to  the  finer-grained  ejectaof 
volcanic  eruptions  ;  may  consist  almost  entirely  of  lapilli  (q.  v.)  or  of  the 
finest  sand  and  dust,  or  of  a  mixture  of  coarse  and  fine  ingredients. 

Unconformable  :  not  conforming  in  position,  or  not  having  the  same  inclination 
or  dip  with  underlying  rocks  ;  applied  to  strata  which  rest  upon  an  eroded 
surface  of  older  rocks  ;  unconformity  or  unconformability,  the  condition  of 
not  being  conformable. 

Underclay  :  the  bed  upon  which  a  coal-seam  rests. 

Uniclinal  (L.  unus,  one  ;  Gr.  klino,  to  lean) :  applied  to  a  series  of  strata  dip- 
ping  in  one  and  the  same  direction. 

Upthrow,  upcast :  that  side  of  a  fault  on  which  the  strata  lie  at  a  higher  level 
than  their  continuations  on  the  other  side  of  the  fault.  Normal  faults  are 
usually  described  as  downthrows ;  reversed  faults  as  upthrows. 

Wady  (Ar.) :  a  ravine  or  watercourse,  dry  except  in  the  rainy  season.     Some 

wadies  are  perennially  dry. 

iVWeatheringr ;  applied  to  the  decomposition,  disintegration,  and  breaking  up 
of  the  superficial  parts  of  rocks  under  the  general  action  of  changes  of  tem- 
perature, and  of  wind,  rain,  frost,  etc. 

Zeolites  (Gr.  zeo,  I  boil ;  Kthos,  stone) :  a  group  of  minerals,  so  called  because 
they  bubble  up  in  the  blowpipe  flame  ;  often  met  with  filling  up  vesicular 
cavities,  etc.,  in  igneous  rocks. 


INDEX 


Aar  Glacier,  215 

Abrasion  by  ice,  216,  241,  248 

Abyssinia,  plateaux  of,  186,  339 

Accumulation-mountains,  340 

Achumore,  97 

yEolian  action,  24,  250 

—  basins,  257,  260,  284 
African  coasts,  328 

—  lakes,  162,  279 
Afton  Water,  138 
Air  volcanoes,  185 
Aix-la-Chapelle,  127 
Akabah,  gulf,  159 
Aletsch  Glacier,  306 
Alluvial  basins,  283 

—  terraces,  7,  49,  50 

Alpine  glaciers,  work  done  by,  213, 
217 

—  lands,  glacial  phenomena  of,  227, 

246,  247 

Alps,  the,  93,  109,  119,  208,  214,  216, 
217,  231,  284,  291,  293,  296,  312, 

35i 
Amazon,  delta,  52 

—  river,  7 
Andes,  cirques,  292 
Andesite,  174,  2pi 

Animals,  geological  action  of,  29 
Annan  Water,  133 
Anticlinal  double-fold,  96 

—  hills  and  mountains,  88, 91, 104,  III 

—  valleys,  10,  85,  86,  112,  116,  117 
Anticlines,    symmetrical,  85,   86,  88, 

90,  105,  112,  115,  117,  119 

—  unsym metrical,  10,  93,  94,  99,  107, 

116,  1 20 

Antilebanon,  162 
Antrim,  basalts,  186,  191 
Appalachian  Mountains,  93,  118 


Aqueous  rocks,  3,  4,  22 
Arabah  Mountains,  256 

—  Wady,  159 

Aralo-Caspian    depression,    52,    279, 

337 

Ardennes  Mountains,  127 
Argillaceous  rocks,  21 
Arizona,  53 

Arkansas,  aeolian  basins  of,  284 
Auvergne,  caves,  275 

—  lakes,  281 
Axial  uplift,  129 

Rahia,  257,  284 

Baltic  Glacier,  247,  306 

—  pay  sage  morainique,  247 
Baltzer,  Prof.,  221 
Bandaisan,  282 

Barrier  lakes,  281,  293,  298,  305 
Basalt,  20,  21,  174 

—  caves,  276 

—  plains  and  plateaux,  186 

—  sea-cliffs,  324 

—  weathering,  26,  201 

Base-level  of  erosion,  59,  63,  66,  87, 

-   140,  143,  149,  226,  343,  360 
Basins,  origin  and  classification,  278, 

279,  359 

Bathgate  Hills,  345 
Bavarian  Alps,  112 
Beach  gravels,  325 
Belgium,  carboniferous  districts,  127 
Ben  Alligin,  147 

—  Dearg,  147 

—  Eighe,  147 

—  Lomond,  142 

—  Muich  Dhui,  142 

—  Nevis,  142 

—  Uidhe,  97 


387 


388 


INDEX 


Berendt,  Prof.  G.,  260 

Berlin,  233 

Bertrand,  Prof.  M.,  95 

Bex,  297 

Bingen,  165 

Birnam,  169 

Black  earth,  263 

—  Forest,  163 
Blind  valleys,  271 
Bohm,  Dr.,  229 

Bottom  moraine,  see  Ground-moraines 
Boulder-clay,  composition  of,  233 

—  configuration  of,  233 

—  marine  erosion  of,  319 
Boulogne,  127 
Bowdoin  Glacier,  224 
Bracciano,  lake,  281 
Brandenburg,  238 
Brazil,  coasts,  329,  330 

—  schistose  rocks,  6 

—  weathered  rocks,  205 
Briart,  M.,  127 
Brick-clay,  21 

British  mountains,  93 
Buttes,  59,  344,  376 

Cairngorm  Mountains,  290 
Caithness  pyramidal  hills,  71 
Calcareous  rocks,  208 
Caldeiraos,  257 
Caledonian  Canal,  144 
Calif ornian  lava-caves,  275 
Cambusnethan,  167 
Canada,  schistose  rocks,  6 

—  lakes,  301 

Canary  Islands,  lava-caves,  275 

Canisp,  71 

Canons  of  Colorado,  53,  66 

Cape  Blanco,  257 

Cape  Bojador,  257 

"  Capture"  by  streams,  108,  122,  131, 

138,  144,  148 

Carinthia,  Karst-regions,  271 
Cam  Chois,  146 
Carpathian  Mountains,  115 
Cascades,  see  Waterfalls 
Caucasus,  119 
Caverns,  31,  209,  269,  272-277,  282, 

325 

Cevennes,  208 
Chalk,  22 

—  escarpments,  83,  84,  345 
Chamberlin,  Prof.,  220,  223 


Changes  of  sea-level,  12,  13 
Chemical  action  of  rain,  25 

—  of  underground  water,  30,  267 
Chilian  Andes,  292 

Chiitern  Hills,  345 
China,  dust  deposits,  261 
Choffat,  P.,  116 
Cinder  cones,  181,  182 
Circumdenudation     mountains,     132 

145,  147,  193,  204,  346 
Cirque  basins,  287 

—  lakes,  286 

—  valleys,  70,  290 
Classification  of  land-forms,  335 
Clermont,  275 

Cliffs,  river-,  61,  68,  72,  76,  353 

—  sea-,  71,  319 

—  undercut  by  wind-action,  24 
Climate,  influence  of,  on  denudation, 

64,  72,  370 
Coal,  4,  23 
Coastal  plains,  326 
Coast-lines,  general  trend,  317,  328, 

361 
Coasts,  indented  or  irregular,  327 

—  smooth  or  regular,  325 
Colorado  Plateau,  344 

—  faults  of,  156 

—  river,  53,  57,  67,  156 
Como,  lake,  293,  298 
Concretions,  256 

Cone-in-cone  structure  of  volcanoes, 

183. 

Conglomerate,  3,  22 
Connel  Water,  138 
Constance,  lake,  293,  298 
Constriction-basins,  299,  302 
Constructional  valleys,  347 
Continental  plateaux,  339 
Coral  reefs,  334 
Cordilleras,  93,  119 
Cornet,  M.,  127 
Cornwall,  sea-caves,  276 
Corrie,  see  Cirque. 
Cotswold  Hills,  83,  345 
Coulmore,  71 
Crag-and-tail,  242 
Crater  lakes,  281 
Cree,  river,  133 
Crevasses  in  glaciers,  216,  218 
Crieff,  192 
Crustal  deformation,   13,  47,  48,  179, 

209,  280,  330 


INDEX 


389 


Crustal  movements,  influence  of,  on 
land-surface,  17,  47,99,  157,  158, 
159,  162,  164 

Crystal  cellars,  275 

Crystalline  schists,  origin  of,  7 

Curve  of  erosion,  357,  377 

Cycle  of  erosion,  65,  125,  140,  148, 
172,  338 

—  interrupted,  125,  135,  149 


Dachstein  glaciers,  221 
Dana,  Prof.,  240 
Danube,  river,  52,  263 
Darmstadt,  164 
Dead  Sea,  159,  162,  279 
Deccan  Plateau,  186,  339 
Decomposition  of  rocks,  25-30 
Dee,  river,  133,  141 
Deflation,  24,  250 
Deflection-basins,  297,  303,  314 
Deformation,  crustal,  13,  47,  48,  179, 
209,  280,  330 

—  mountains,  341 

—  valleys,  350 

De  Geer,  Baron,  234 
Deltas,  49,  52 

—  growth  of,  37 

—  structure  of,  49 

Denmark,  thickness  of  ice-sheet,  233 
Denudation,  agents  of,  19 

—  estimates  of  rate  of,  38,  370 

—  evidence  of,  12,  13 

—  in  limestone  regions,  270 

—  land-forms     assumed     under,    de- 

pendent on  various  factors,  45 
Depressed  areas,  159,  162 
Derivative  rocks,  5,  6,  12 
Deserts,  251 

—  regular  coasts  of,  328,  333 
Diablerets,  113 

Dikes,  20,  173,  176,  180,  191,  276 
Diluvial  doctrine,  2 
Dip,  8 

—  slopes,  73,  77,  254 
Discontinuity   of  strata,   evidence  of 

denudation,  14 

Disintegration  of  rocks,  23-25,  199 
Dislocation  mountains,  342 
Dislocations,  see  Faults. 
Dissolution  basins,  282 
Dolinas,  271,  273 
Dolomite  mountains,  72 


Dombes,  paysage  morainique,  300 
Dome-shaped  hills,  6 
Dome-shaped  strata,  erosion  of,  74 
Doon,  river,  133,  138 
Double-folds,  96,  115 
Downs,  85,  345 
Downthrow  side  of  faults,  155 
Drainage,  modifications  of,  by  glacial 

action,  355 

—  underground,  31,  268 
Drumlins,  234,  245,  378 
Drummond  Castle,  192 
Drums,  234,  245,  378 
Drygalski,  Dr.,  220 
Dry  valleys,  252,  271 
Dunes,  258,  259 
Dust  of  deserts,  260 
Button,  Capt.,    58,   62,   63,   66,  67, 

158 
Dykes,  see  Dikes. 


Early  views  as  to  origin  of  land-forms,  I 

Earn,  valley,  169 

Earthquakes,  164,  267 

East  African  lakes,  162 

Eifel,  281 

Elevation  mountains,  102 

Elk  Mountains,  342 

Engadine,  243,  284 

English  Channel,  317 

Epigene  agents,  4,  23 

—  general  results  of  their  action,  332 

—  influence  of,  in  land-sculpture,  46 
Erosion  of  anticlines,  105 

—  of  arid  regions,  206 

—  of  calcareous  regions,  268 

—  of  Grand  Canon  district,  57 

—  of  horizontal  strata,  49 

—  of  inclined  strata,  75 

—  of  mountains  of  uplift,  125 

—  of  volcanic  accumulations,  187 

—  factors  determining  results  of,  45 

—  fluviatile,  35 

—  glacial,  215,  287,  293,  300,  311 

—  marine,  316 

—  rate  of,  38,  370 

—  valleys  of,  347 

—  various  processes  of,  23 
Escarpment  mountains,  146 
Escarpments,  73,  76,  79,  82,  84,  88, 

120,  254,  304,  343 
Escher,  Von,  221 


390 


INDEX 


Eskers,  245,  249,  378 
Ettrick,  river,  139 
European  ice-sheet,  232 
Evolution  of  land-forms,  3,  365 

Factors  of  erosion,  46 
Falls  of  Clyde,  355 
— •  Niagara,  254 
Fan-shaped  structure,  96 
Faroe  Islands,  68,  186,  344 
Faults,  bounding  Scottish  Lowlands, 
167,  168 

—  cavities  in,  276 

—  coal-fields,  95,  127,  155,  166,  167 

—  Colorado  region,  156 

—  connection  of  volcanoes  with,  185 

—  East  African  lakes,  162 

—  evidence  of  rock-removal,  15,  158 

—  Great  Basin,  157 

—  influence  on  surface,  150 

—  Jordan  Valley,  159 

—  normal,  12,  48,  98,  150 

—  related  to  flexures,  152 

—  reversed,  94 

—  Rhine  Valley,  163 

Fauna  of  steppes  and  tundras,  263 
Felspars,  20,  25,  378 
Felspathic  rocks,  20 
Finland,  6 

—  glacial  erosion,  235,  239 

—  lakes,  286,  301 

—  moraines,  246 
Fiord  basins,  307 

—  coasts,  328,  329 
Fissure  eruptions,  185,  189 
Fjelds,  Norwegian,  308 
Flexures,  mountain,  99 

—  symmetrical,  118,  119 

—  unsymmetrical,  116 
Floods,  river,  33 

Fluvio-glacial  deposits,  226,  237,  249, 

263 

Fluvio-marine  deposits,  49 
Folded  mountains,  101,  341 

—  strata,  9 

Folding,  cause  of,  13,  48,  236 
Folds,  disrupted,  94 

—  influence  of,  on  surface,  101 

—  isoclinal,  93,  94 

—  symmetrical    and     unsymmetrical, 

116 

—  varieties    of,    in    deeply    inclined 

strata,  93 


Fox,  Arctic,  264 

Fraas,  Dr.  E.,  112-114 

Fragmental    igneous    rocks,    4,    179, 

182,  187,  189 
Freiburg,  164 
Frost,  action  of,  28 
Frtth,  Dr.,  234 


Gabbro,  174 
Galloway,  drumlins,  234 
Ganges,  river,  52 
Garda,  lake,  293 
Gavarni,  cirque,  290 
Geneva,  lake,  36,  293,  296 
Geological  structure,  influence  of,  in 
denudation,  45,  48,  86,  124,   209, 

319 
Germany,  cirques,  291 

—  glacial  deposits,  235,  238,  244-247 
— paysage  morainique  and  lakes,  286 
Geysers,   185 

Giant's  Causeway,  21 
Gibraltar,  208 

Gilbert,  G.  K.,  159,  176,  284 
Girvan,  168 

Glacial  accumulations,  225,  233,  243, 
248,  301,  305 

—  action,    land-forms    modified    by, 

211,  212,  241,  248 

—  basins,  285 

—  erosion,    215,    220,   224,    235-240, 

248,  287,  292,  298,  303 

—  rivers,  215 
Glaciers,  Alpine,  213 

—  geological  action  of,  213,  220 

—  Norwegian,  214,  217 
Glarus,  Canton,  115 
Glassy  rocks,  19 
Glasven,  97 
Glenbeg,  97 

Glen  Docherty,  145 

—  Eunach,  290 

—  Garry,  145 

—  Lyon,  146 

—  Roy,  306 
Glenmore,  141,  142 
Glutton,  264 
Gneiss,  23,  26 
Gorges,  origin  of,  81 
Grabiinden  Alps,  291 
Grand  Canon  district,  53,  66 
Granite,  aeolian  erosion  of,  252 


INDEX 


Granite,  joints  in,  200 

—  lava- form  equivalent  of,  174 

—  mountains,  175 

—  plains,  175 

—  presence   of,  at  surface,   evidence 

of  denudation,  16,  174 

—  weathering  of,  26,  201 
Granitoid  rocks,  weathering  of,  206 
Gravel-and-sand  rocks,  21 

Great  Basin  ranges,  157,  341 
Greenland,  ice  of,  214,  220,  224 
Green  River,  89,  90 
Grindelwald  Glacier,  222 
Ground-moraines,  214 

—  Alpine,  228 

—  source,  220,  228,  233 

—  superficial  form,  244 
Gumbel,  Dr.,  113 
Gypsum,  23,  267,  268 


Hallstadter  See,  290 
Hawaii,  183,  275 
Hebrides,  Inner,  186,  191 

—  Outer,  243,  301,  303 

Heim,  Prof.  A.,  no,  114,  116,  216, 

221,  231 

Helensburgh,  168 
Helland,  A.,  215,  238,  292 
Henry  Mountains,  177,  342 
Hesse,  165 
Highlands  (Scotland),  corries,  289 

—  geological  structure,  140 

—  hydrographic  system,  141 

—  lake  basins,  289,  291,  293,  301 

—  plateau  of  erosion,  140 

—  pyramidal  mountains,  71 

—  relict  mountains,  145 

—  thrust-planes,  97 
Hills,  339  ;  see  Mountains. 
Himalaya,  93,  119,  216,  292 
Hinman,  R. ,  159 

Hohe  Tatra,  291 

Hohe  Tauern,  223 

Hoist,  Dr.,  220 

Holstein,  306 

Horizontal  strata,  8,  49,  52,  59,  319 

Hornblende,  20 

Hornkees  Glacier,  223 

Horste,  170,  342,  380 

Huron,  lake,  279 

Hypogene  action,  47 

—  rocks,  4 


Ice  Age,  modification  of  pre-glacial 
drainage-systems  during,  355 

Ice-barrier  basins,  306 

Iceland,  185,  215,  275,  344 

Ice-sheet  of  Europe,  232 

Igneous  action,  land-forms  due  to, 
173,  193 

—  rocks,  4,  19,  26,  197,  200,  324 
Illyria,  271 

Infraglacial  accumulations,  213,  220, 

227,  238,  380 
Ingleborough,  344 
Inland  ice  of  Northern  Europe,  232, 

300 

Inn  Glacier,  229 
Innsbruck,  229 
Insolation,  23,  250 
Insoluble  residue  of  calcareous  rocks, 

26 

Intraglacial  detritus,  238,  380 
Inversion,  n,  114 
Ireland,  sea-caves,  276,  277 
Ironstone,  23 
Isar  Glacier,  230 

Islands,  fringing  or  marginal,  328 
Isoclinal  folds,  93,  94,  116,  129 
Issyk-Kul,  279 
Italy,  volcanic  lakes,  281 

Jerboa,  264 
Jessero,  lake,  273 
Joints  in  rocks,  21,  22 

—  influence  of,   in  erosion,    60,   72, 

105,  197,  319,  320 
Jordan  Valley,  159 
Jostedalsbrae,  290 
Jura  Mountains,  115,  208,  297 
Jurassic  escarpments,  83 
Justedal  Glacier,  215 

Kaisergebirge,  112 
Kaiserstuhl,  164 
Karls-Eisfeld,  221 
Karrenfelder,  208 
Karst  regions,  271 
Keilhack,  Dr.,  234 
Ken,  river,  137 
Kettle  valleys,  271,  272 
Kinnaird  Point,  142 
Konigs  See,  290 
Kopjes,  206 
Kurisches  Haff,  260 


392 


INDEX 


Laccoliths,  176,  342 

Lac  d'Aydat,  281 

Lacustrine  deposits,  49 

Ladoga,  lake,  279,  302,  361 

Lake-basins,  irregular  depths,  299 

Lake-lands,  301 

Lakes  as  settling  reservoirs,  36 

—  formed  by  river  action,  283 

—  in  cirques,  287 

—  in  deserts,  257,  260 

—  in  glaciated  lands,  285 

—  in  limestone  regions,  272,  282 

—  in  moraines,  300 

—  in  mountain  valleys,  292,  299 

—  in  Scottish  Highlands,  312 

—  in  steppes,  264 

—  in  tectonic  basins,  279 

—  in  volcanic  regions,  281 

—  silted  up,  7 

—  temporary,  273 

—  vertical  distribution  of  high-level, 

291 
Lanarkshire,   faults  in  coal-fields  of, 

166 

Landes,  French,  337 
Land-forms  due  to  glacial  action,  241 
Lava,  4,  20 

—  caves  in  and  underneath,  274 

—  cones  of,  182 

—  plutonic  equivalents  of,  174 
Leader,  river,  138 
Lebanon,  162 

Lee-seite,  222,  381 

Lemming,  264 

Lewis,  303 

Libyan  Desert,  24,  254,  256, 

Lignite,  4,  23 

Limestone,  22 

—  Alps,  113 

—  underground  drainage  in,  268 

—  weathering  of,  207 
Llanos,  337 
Llathach,  147 
Lochaber  Mountains,  147 
Loch  Ewe,  301 

—  Laxford,  301 

—  Lochy,  141 

—  Lomond,  293,  313 

—  Maree,  145,  146,  313 

—  Ness,  293,  312 

—  Torridon,  147 
Loss,  240,  261 

Lombardy,  moraines  of,  247,  296 


Longitudinal  valleys,  76,  80,  104,  122, 
139,  145,  148,  350;  see  Strike- 
valleys. 

Lothians  (Scotland),  245 

Lowland  basins,  300 

Lowlands  (Scotland),  land-forms  in, 
344 


Maars,  281 
Macalubas,  185 
Madagascar,  329 
Marjelen  See,  306 
Maggiore,  lake,  293 
Maiden  Pap,  71 
Malvern  Hills,  82 
Mangrove  Swamps,  333 
Marble,  22 
Marl,  22 
Marmots,  264 
Mashonaland,  206 
Massive  eruptions,  185 
Mauna  Loa,  183 
Mazellferner  Glacier,  223 
Mecklenburg,  238,  306 
Merse,  137 

Mesas,  59,  67,  344,  376 
Metamorphic  rocks,  5 

—  presence  at  surface  proves  denuda- 

tion, 16 
Mica,  20 

—  schist,  23 

Michigan,  lake,  279,  361 
Midlands,  escarpments  of,  84 
Minerals,  common  rock-forming,  20 
Minto  Hill,  190 

Mississippi,  river,  7,  38,  52 
Moab,  mountains  of,  161 
Monadhliath  Mountains,  147 
Mongolia,  seolian  basins,  284 
Monoclinal  folds,  54 
Mons,  coal -basin,  95 
Monte  Somma,  184 
Moor  of  Rannoch,  142 
Moors,  Yorkshire,  345 
Moraines,  lateral,  246 

—  superficial,  213,  214,  216 

—  terminal,      219,    246,    249,     303 

301 

Morainic  lakes,  300 
Moray  Firth,  141,  312 
Morven,  71 
Moulins,  217,  381 


INDEX 


393 


Mount  Ellen,  178 

—  Ellsworth,  178 

—  Killers,  178 

—  Holms,  178 

—  Pennell,  178 
Mountains,  accumulation,  186 

—  anticlinal,  erosion  of,  104 

—  circumdenudation,   58,  65,  67,  69- 

72,  76,  79,  83,  86,  88,   132,  145, 
147,  193,  204,  343,  346 

—  classification  of,  339 

—  contrast   between  young  and  old, 

100,  125 

—  demolition  of,  123,  125 

—  escarpment,  146 

—  subsequent  or  relict,  145 

—  upheaval,  formation  of,  101 

—  various  ages,  92,  93 

—  young,  relatively  unstable,  119 
Mountain-track  of  rivers,  35,  377 
Mountain-valley  basins,  292 
Mud  volcanoes,  185 
Mushroom-shaped  rocks,  24,  253 
Musk-ox,  264 

Nahr  el  Asi,  162 
Necks,  189,  191,  345 
Ness,  loch,  141 
Neuchatel,  lake,  296 
Neumark,  306 
Neve,  227,  290,  310,  382 

—  line,  289,  291 
Newcastle  coal-field,  167 
New  Zealand,  216,  292,  330 
Niagara,  354 

Niger,  river,  52,  257 

Nile,  river,  7 

Nith,  river,  133 

Nithsdale,  233 

North  America,  glacial  deposits,  244 

—  ice-sheet,  232,  240 

—  lakes,  279,  301 

—  pay  sage  morainique,  306 
North  Sea,  317 
Norway,  cirques,  292 

—  cirque-valleys,  290 

—  fiords,  233,  307 

•  —  glaciers,  214,  217 
Nunatakkr,  220,  227,  233,  314 

Oases,  252 

Obersalzbachkees  Glacier,  223 


Obsidian,  20 

Oceanic  basin,  328 

Ochil  Hills,  88,  345 

Oetzthal,  229 

Old  Red  Sandstone  mountains,  71 

Olivine,  20 

Onega,. lake,  279,  302,  361 

Original  mountains,  340 

—  valleys,  347 

Orkney,  sea-caves,  276 

Orontes,  river,  162 

Osar,  245 

Outer  Hebrides,  243 

Outliers,  84,  382 

Overfolds,  94,  113,  114 

Overthrusts,  94,  115 


Palestine,  mountains,  161 

Pampas,  337 

Parallel  roads,  306 

Partsch,  Prof.,  291,  292 

Paysage  morainique,  247,    286,    300, 

306 

Peat,  4 
Penck,  Prof.,  38,  164,  221,  222,  230, 

282,  291,  326 
Pennsylvania,  118 
Pentland  Hills,  345 
Permian  Basin,  Ayrshire,  85 
Pernambuce,  332 
Perth,  192 
Peruvian  Andes,  292 
Phonolito,  201 

Piedmont,  moraines,  247,  296 
Pitchstone,  20 
Plains,  classification  of,  335 

—  of  accumulation,  49,  186,  326,  335 

—  of  erosion,  127,  128,  136,  142,  186, 

337 

Plain-track  of  rivers,  35,  377 
Planina,  river,  273 
Plants,  geological  action,  29 
Plateau,  basins,  300 

—  continental,  327 

—  Norwegian,  308 

—  Scottish  Highlands,  142 

—  Southern  Uplands,  Scotland,  133 
Plateaux,  accumulation,    52,   60,   65, 

1 86,  338,  343 

—  classification  of,  338 

—  denudation   of,   60,   65,    131,    13* 

137,  MI,  147 


394 


INDEX 


Plateaux,   direction   of    drainage    in, 
130,  141,  148,  351 

—  erosion,  77,  78,  129 

—  surface  inclined  against   dip,   80, 

—  surface  inclined  in  direction  of  dip, 

77 

Plate,  river,  332 
Plutonic  rocks,  4,  173 

—  lava-form  equivalents  of,  173 

—  presence  at  surface  implies  denuda- 

tion, 16,  174 
Poland,  moraines,  247 
Pomerania,  moraines,  306 
Po,  river,  7,  37,  52 
Posen ,  238 

Powell,  Major,  54,  89,  91,  189 
Prairies,   337 

Pre-Cambrian  sandstone  mountains,  71 
Prehistoric  glaciers,  225 
Prussia,  glacial  deposits,  238,  306 
Pumpelly,  Prof.,  284 
Pyramidal  hills  and  mountains,  58,  65, 

68-72,  194,  344 
Pyrenees,  290,  292 
Pyroxene,  20,  383 


Quadersandstein,  72,  206,  383 
Quarrying,  infraglacial,  221 
Quartz,  20 
Quartz-rock,  22 
Queantoweep  Valley,  158 
Quinaig,  97 


Rain,  action,  25,  32 
Raised  beaches,  49,  277 
Ramsay,  Sir  A.  C.,  85,  294 
Raniaka  Cave,  275 
Rapids,  8 1 

Rat,  little  hamster,  264 
Reclus,  E.,  327 
Red  Sea,  162 

Regional  elevation,  128,  129 
Regular  coasts,  318 
Reindeer,  264 
Relict  mountains,  342 
Renevier,  Prof.,  113,  114 
Reversed  faults,  94 
Rhine  Valley,  163,  263 
Rhone,  delta,  37,  52 

—  river,  36 

—  valley,  296,  300 


Rias,  313 

Richter,  Prof.,  308-310 

Richthofen,  Baron,  261 

Rio  Janeiro,  330 

River  cliffs,  recession  of,  61,  63 

Rivers,  change  of  course,  108 

—  direction  of,  not  influenced  by  faults 

and  flexures,  57,  156,  165 

—  erosion  by,  59 

—  flowing  in  direction  of  dip,  77,  80 

—  flowing  in  direction  of  strike,  75  \/ 

—  geological  action  of,  34 

—  longitudinal,  107 

—  older  than  mountains  they  traverse, 

46,  57 

—  original  course,  determined  by  sur- 

face-slope, 56,  74,  77,   104,  120, 

131,  137,  351,  353 

—  terraces  of,  7,  51 

—  underground,  268 

—  valleys,  eroded  by,  350,  357 
Roches  moutonne'es,  222,  2 34,  288,  301, 

383 

Rock  basins,  222,  288,  293 
Rock-fall  basins,  284 
Rock-falls,  119 

Rock-flexures,  infraglacial,  236 
Rock-flour,  215 
Rock-forming  minerals,  20 
Rock-removal,  evidence  of,  13 
Rock-salt,  23,  266 
Rock-shattering,  infraglacial,  221 
Rock-shelters,  276 
Rocks,  chemical  composition  of,  20 

—  classes  of,  3,  4,  19 

—  comparative  resistance  of,  to  denu- 

dation, 44,  77,  78 

—  disintegration  of,  23-26,  198 

—  porosity  of,  21 

—  shattered  by  frost,  29 
Rodgers,  Prof.,  118 
Rotted  rock,  27 
Rubers  Law,  136 
Rttgen,  234 

Rule  Water,  136 
Rumpfgebirge,  170,  380 
Russia,  black  earth,  263 

—  ground-moraines,  238 

—  lakes,  286 


Saddlebacks,  see  Anticlines. 
Sahara,  251 


INDEX 


395 


St.  Gall,  canton,  115 

Salses,  185 

Salt  Lake,  Utah,  279 

Sand-blast,  natural,  24 

Sand,  blowing,  253 

Sand  hills,  259,  325  ;  see  Dunes. 

Sandstone,  4,  21 

Sand  wastes,  257 

Santa  Marta  (Sierra  Nevada),  292 

Saxon  mountains,  344 

—  Switzerland,  72,  206 
Scandinavia,  glacial  erosion,  235 

—  glaciers,  217 

—  ice-sheet,  232,  233 

—  moraines,  246 

—  mountains,  93 

—  plateau,  130 
Schists,  6,  20 

—  jointing  in,  197,  199 

—  marine  erosion  of,  324 

—  presence  at  surface  implies  denu- 

dation, 16 

—  weathering  of,  204,  205 
Schleswig-Holstein,  245,  246 
Schortenkopf,  112 

Scoriae,  182,  384 

Scotland,  corries  and  cirque  valleys, 
290 

—  thickness  of  ice-sheet,  233 
Scottish  Highlands,  6,  141 
Screes,  29,  205,  229,  384 
Sea,  caves,  276 

—  cliffs,  317 

—  floor,  subsidence  of,  12 

—  geological  action,  317 

—  lochs,  307 
Sedimentary  deposits,  4,  6 

—  rocks,  22 

—  strata,  average  thickness  of,  43 
Sediment  of  glacial  rivers,  214 
Senegal,  river,  257 

Severn,  river,  83 
Shale,  3,  21 

Shearing  of  rocks,  48,  95,  99 
Sheets,  intrusive,  20,  173,  177 
Shell  marl,  4 
Siberia,  52,  264 
Sidlaw  Hills,  345 
Sierra  el  Late,  203 

Sierra   Nevada    (Santa   Marta),    157, 
292 

—  (Spain),  292 
Silicious  rocks,  21 


Silser  See,  284 

Silvaplana  See,  284 

Simony,  Prof.,  221 

Sinai  Peninsula,  256 

Sink-holes,  282 

Slags,  182 

Smean,  71 

Smooth  coasts,  318 

Snow,  action  of  melting,  32 

Snow-line  in  Alps  during  glacial  pe- 
riod, 227 

Soils,  waste  of,  34 

Somma,  Monte,  184 

Sounds  of  Faroe  Islands,  71 

Southern  Uplands  (Scotland),  129, 
133,  289 

Sowbacks,  245 

Spain,  rias  of,  313 

Spey,  river,  141,  145 

Springs,  influence  of,  in  valley-erosion, 
76 

—  mineral,  267,  274 

—  natural,  31,  105 
Sserir,  256 

Staff  a,  21 

Stampflkees  Glacier,  214,  223 

Steppes,  263,  337 

—  fauna  of,  264 
Stinchar,  river,  134 
Stonehaven,  168 
Stoss-seite,  222 

Strata,  discontinuity  of,  evidence  of 
erosion,  15 

—  gently-inclined,  denudation  of,  73, 

319 

—  highly-folded,    denudation  of,  92, 

322 

—  horizontal,  denudation  of,  49,  319 
Striae,  glacial,  288 

Striated  stones,  214 

Strike-basins,  304 

Strike-valleys,  76,  80,  131,  352  ;   set 

Longitudinal  valleys. 
Submarine  basins,  306 
Subsequent  mountains,  342 

—  valleys,  347 
Suess,  Prof.,  162 
Suilven,  71 

Summit  glaciers,  214,  217,  290 
Superior,  lake,  279,  361 
Sutherland,  mountains,  344 
Swallow-holes,  208 
Sweden,  glaciated  areas,  239 


INDEX 


Sweden,  osar,  245 

syenite,  174 

Synclinal  double-fold,  96 

—  hills    and    mountains,    10,   86-88, 

344 

—  valleys,  89,  104-107 

Synclines,    symmetrical,     10,    86-89, 
105,  112,  115,  117,  II8 

—  unsymmetrical,  10,  42,  93-9°.  99' 

107,  no,  112-114 
Systems,  geological,  5 

—  united  thickness  of,  5 

Table-lands,  338 

—  mountains,  254,  344 
Tailless  hair,  264 
Tarbat  Ness,  141 
Tarns,  288 

Tay,  river,  141,  145 

—  valley,  190 
Tectonic  basins,  279,  360 

—  mountains,  340 

—  valleys,  347 
Teith  Valley,  169 
Terraced  mountains,  70 
Terraces,  alluvial,  51 

—  marine  erosion,  321,  331 
Teviotdale,  234 

—  river,  136,  139 
Thames,  river,  84 

Thickness  of  sedimentary  strata,  43 

Thrust-planes,  95,  114 

Tiberias,  lake,  159 

Timan  Mountains,  232 

Torrents,  action  of,  289 

Tors  of  Cornwall,  206,  384 

Trachyte,  201 

Transport    of     weathered     materials 

34 
Transverse    streams,    104,    107,    121 

131,  139.  J48^  J49 

—  valleys,  350 

Transylvanian  Alps,  cirques  of,  292 
Trenta,  cirque-valley,  290 

Tuff,  20 

—  cones,  181 
Tundras,  52,  263,  337 
Tweeddale,  233 
Tweed,  river,  133.  ^ 
Tynedale  fault,  167 

Uckermark,  moraines  of,  306 


Jebergossenen  Aim,  221 

Jnconformity,  42,  385 

Jnderground    water,    action    of,    30, 

266 
Uniclinal  orographic  blocks,  159 

Jpcast  side  of  faults,  155 

Jplift,  mountains  of,  101 
—  regional  and  axial,  129 
Utah,  53,  177,  279 


Vacek,  Dr.,  116,  117 
Valais,  268 
Val  d'Uina,  112 
Valley-track  of  rivers,  35,  377 
Valleys,  Alpine,  308 

—  classification  of,  347 

—  constructional,  347 

—  deformation,  348 

—  dislocation,  159,  162 

—  erosion,  145,  349 

—  in  gently-inclined  strata,  75 

—  in  highly-folded  strata,  104 

—  in  horizontal  strata,  60 

—  longitudinal,  70,  131,  139.  T44 

—  older  than  mountains  they  traverse, 

46,  57 

—  submerged,  329 

—  subsequent,  349 

—  synclinal,  104,  105,  121 

—  transverse,  104,  122,  131,  139,  144 

—  U-shaped  and  V-shaped,  308 

—  variations  in  form  of^  353 
Vatnajokull,  215 

Veins,  20 
Vesuvius,  184 
Vispthal,  267 
Volcanic  basins,  281 

—  rocks,  4,  173 
Volcanoes,  180 

—  demolition  of,  187 
Vorlander,  Alpine,  238,  246,  296 


Wadies,  25,  159 

Wahnschaffe,  Dr.,  238 

Wahsatch  Mountains,  157 

Wallace,  Dr.,  311 

Wallenstadt,  mountains,  114 

Walther,  Prof.,  24,  254,  256 

Water,  chemical  action  on  rocks.  25, 

30 
—  mechanical  action,  33 


INDEX 


397 


Waterfalls,  80,  355 

Wealden,  anticline,  85 

Weathering  of  rocks,  26,  199 

West  Lomond  Hill,  88 

Whiteadder,  river,  138 

W7ind,  geological  action  of,   24,  252, 

265 

Wocheinerthal,  290 
Wolds,  Yorkshire,  345 


Yarrow,  river,  139 
Yellowstone  Lake,  281 

Zambesi  Falls,  357 
Zillerthal,  214,  223 
Zirknitz,  273 
Zmutt  Glacier,  221 
Zones  of  cirques,  291 
Zurich,  lake,  293 


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